"Therefore my Lord himself will give you a sign. Look, the concealed one shall conceive and bear a son, and she shall call his name Immanuel" (Isa 7:14).
The historical context of this important prophetic saying, actually, one of the most popular messianic texts in the Christian interpretive tradition, may be summarized in a few words. Isaiah of Jerusalem, the 8th century BC prophet, approaches king Ahaz of Judah on the eve of the so-called Syro-Ephraimite war (733-732 BC). The king of Israel, Pekah, had joined Rezin of Damascus in his revolt against Assyria, and together they were heading to Jerusalem, in order to depose Ahaz, a client of Assyria, and replace him with a certain ben tab'al "son of Tabeel" (2 Kgs 15:37; 16:5; Isa 7:6). Isaiah's message to Ahaz was quite simple: Trust God rather than all weaponry and any measure of defense because all man-made remedies are meant to fail. Yet, instead of listening to Isaiah's warning (Isa 7:4f.), Ahaz appeals to the Assyrian king Tiglath-pileser III sending him gifts from the temple in order to remain a king (2 Kgs 16:7-8). Assyria intervenes promptly. Damascus and Samaria are attacked and territorially diminished, while Ahaz's Judah was reduced to vassalage.
The text to be discussed here, Isaiah 7:14, is part of the so-called Book of Immanuel (Isa 6:1-12:6), that gathers prophecies of Isaiah pertaining to the Syro-Ephraimite war. In his approach to king Ahaz, Isaiah gives him the chance to ask God for a "sign" as a warranty that divine protection is an immediate, concrete reality, "Do ask a sign from Yahweh your God, as deep as Sheol or as high as above" (Isa 7:11). In the context of his meticulous preparations for war, Ahaz's refusal to ask God for a sign sounds rather hypocritical, "I will not ask. I will not test Yahweh" (Isa 7:12). In spite of the refusal, the sign is revealed, namely, in the above quoted prophecy. The sign refers to birth of a child, whose prophetic name Immanuel ("God [is] with us") signifies that Yahweh is still with his people and that he will protect Judah. The close relationship between Isaiah and God whom he serves is underscored by the title ‘adonay "my Lord" used by the prophet (Isa 7:14), and throughout the Book of Immanuel Isaiah is more detailed and precise with respect to God's blessings upon Judah via this wonderful child (see Isa 9:1-6; 11:1-9). These prophetic sayings may be listed under the rubric royal messianism already found in Nathan's prophecy (2 Sam 7), and subsequently reworked by Mic 4:14-5:1; Ezek 34:23; and Hag 2:23. The center of the so-called royal messianism is occupied by the belief in the permanence of the Davidic dynasty.
The text of Isaiah 7:14 opens with the interjection hinneh "behold, look, there is" which announces something out of the ordinary, a new moment in the unfolding story (cf. Gen 29:6; Num 25:6), or which calls attention to the following noun or phrase (cf. Gen 15:17; 31:51). In the context of Isaiah 7 the second function seems more likely. Thus, the emphasis falls on the noun following the interjection, namely, on ha-‘almah. The divine initiative in offering a sign of Yahweh's willingness to protect and bless Judah is emphasized by the personal pronoun hu' "he, himself" in the phrase "my Lord himself."
With respect to the Hebrew noun ‘almah,  the editors of HALOT  list among its meanings: "marriageable girl," "a girl who is able to be married," and "a young woman" (until the birth of her first child). The basic meaning is a woman (the age is less important) ready (able) to be married. The span of life covered by this term is poorly defined and quite long, ranging from the onset of puberty to the birth of a woman's first child. 
We propose a different etymology, namely, to derive the noun ‘almah from the root ‘-l-m I "to be concealed, hidden," well attested in Hebrew. If this etymology proves to be correct, ‘alem (masculine) and ‘almah (feminine) would designate an engaged couple, which would accordingly be rendered as "the concealed ones." During the period of betrothal, fiancés used to live in their parents' homes, separated, secluded, forbidden from seeing one another. The feminine form, ‘almah, may also be rendered "the concealed one" or even "the veiled one." This last rendition would reflect the custom of engaged women wearing veils over their faces as a sign of seclusion, or concealment, during the time of betrothal. We may mention that, given the ethical standards of the ancient Israelite society, the idea of virginity, though not distinctly stated, is nevertheless implied in the term ‘almah. As is the case concerning the providential woman from Genesis 3:15 (ha-‘ishshah "the woman"), the noun ha-‘almah "the concealed one" from Isaiah 7:14 has the definite article attached, which points to a special female character, chosen by God from the very beginning to become the mother of Messiah. 
The word ‘ot designates a "sign" confirming the truth of an earlier statement (Exod 3:12; Ju 16:17; 1 Sam 2:34). In Isaiah's case, the prophecy is a sign of confirmation of Isaiah claims in 7:7-9 regarding the fate of Judah's enemies. Interestingly, the pregnancy and birth are described by means of an adjective (harah "pregnant") and an active participle (yoledet "the one who gives birth"), both forms being quite loose in terms of timing. The prophecy is intentionally ambiguous and evasive with respect to the precise time when these events will occur. Everything is wrapped in mystery and cautiously disclosed.
According to the Hebrew text, it is the mother who names her son ("she shall call"), while the Septuagint reads "you shall call," which refers to the king Ahaz's invitation to name the child. The Hebrew version underscores the role of the mother in giving a name to her son, while hinting at the absence of the physical father.  Regarding the identity of the child Immanuel, we assert that the ancient and modern hypotheses which assume that Isaiah had in mind a son of king Ahaz (e.g., the future king Hezekiah) seem unlikely from a chronological (and linguistic) point of view.
The name Immanuel encapsulates a similar theology as the tetragrammaton YHWH (Yahweh) revealed to Moses (Exod 3:13-15). Yet more than the unfinished name YHWH ("He is...."), that emphasizes God's unconditioned, mysterious, always dynamic being, and his perpetual openness to man's concerns, the name Immanuel  points to a one-to-one, personal relationship between God and his people. The latter name also underscores God's willingness to descend to a place among us, to share our human destiny with us, to be one of us. This name is a step foreword in the gradual unfolding of the mystery of the ‘One who is'  on the path towards Incarnation.
"Now the birth of Jesus the Messiah took place in this way. When his mother Mary had been engaged to Joseph, but before they lived together, she was found to be with child from the Holy Spirit.... All this took place to fulfill what had been spoken by the Lord through the prophet, ‘Look, the virgin shall conceive and bear a son, and they shall name him Emmanuel,' which means, ‘God is with us'" (Mt 1:18, 22-23).
"In the sixth month the angel Gabriel was sent by God to a town in Galilee called Nazareth, to a virgin engaged to a man whose name was Joseph, of the house of David. The virgin's name was Mary..... The angel said to her, ‘Do not be afraid, Mary, for you have found favor with God. And now, you will conceive in your womb and bear a son, and you will name him Jesus'" (Lk 1:26-27, 30-31).
Matthew insists that Mary was engaged to Joseph when she conceived her son. Then, he cites the Septuagint's version of the Isaianic text, with a slight modification, "they shall call," instead of "you shall call." He also provides a translation of the name Immanuel. Note that the same Matthew cites the Hebrew text of Hosea 11:1 when he speaks of the return of the infant Jesus from Egypt after Herod's death (Mt 2:15). The Hebrew text of Hosea 11:1 has the singular "my [i.e., God's] son (beni)" which fits quite well in Matthew's Christological application of this text, whereas the Septuagint version reads "his [i.e., Israel's] children (ta tekna autou)," with explicit reference to Israel as a community of people. As one can notice, Matthew was at ease with both Hebrew and Greek versions of the Scripture. Why, then, does he not cite the Hebrew version (ha-‘almah "the concealed one") in Matthew 1:23, which would have made more sense in the context of his story about a betrothed Mary? One explanation may be that the quotation from the Septuagint of Isaiah 7:14 (he parthenos "the virgin") was inserted later into the Gospel of Matthew. Another explanation might be that initially Matthew wrote the gospel in Aramaic, with Isaiah 7:14 in Hebrew or Aramaic translation and a later disciple translating Matthew's Gospel into Greek automatically used the Septuagint version without paying attention to his master's emphasis on Mary's betrothal to Joseph, and without mention of Mary's virginity. However, Matthew explains to a likely non-Hebrew audience unaware of Jewish customs, that, though betrothed, the fiancés were not living together, that Jesus' birth was without a physical father (cf. Mt 1:25). It is interesting to note that the word "virgin" (parthenos) with respect to Mary appears in Luke 1:27, perhaps under the Septuagint's influence, but not in Matthew, while the brief note "betrothed" appears in both gospels. One may deduce that the most common feature of Jesus' birth tradition was Mary's betrothal to Joseph, and that the miracle of Jesus' conception occurred during this betrothal period. We might add that Luke 1:31 cites freely from Isaiah 7:14 replacing Immanuel, the prophetic name of Messiah, with Jesus, its historical counterpart.
If the Hebrew text of Isaiah 7:14 would have had the word betulah "virgin,"  then it would have been a quite general reference: Jesus' conception will occur in the life of a grown-up girl with no sexual experience, most likely not yet engaged (cf. Exod 22:15). This can not be the case if we try to understand the entirety of the Christian tradition of Christ's birth as a fulfillment of Isaiah's prophecy. On the contrary, if we consider the Hebrew text to be the closest to the autograph, then the miracle of conception will happen to ‘almah, an unmarried woman prior to her first birth, thus including the period of engagement. Assuming that our proposed etymology is correct,  the unmarried woman mentioned in Isaiah 7:14 might have been a woman betrothed at the moment of her child's conception. The Hebrew version corresponds much better to what Matthew 1:18 and Luke 1:27 had to say about Mary; namely, that she was "betrothed" to a man, named Joseph, when the annunciation, or the conception, occurred. We might mention at this juncture that Matthew's characterization of Joseph as the "husband" (aner) of Mary should be understood in terms of Joseph as the "fiancé," as which, given the holiness of the betrothal institution, he should be considered de iure Mary's husband (Mt 1:19).
Finally, we should also mention that there is important theological significance to be extracted from both evangelists' observation that Jesus' conception occurred during the betrothal period. In ancient Israel, betrothal was considered a sacred institution which existed only as a free covenant between a man and a woman. The Hebrew text of Isaiah 7:14 alludes prophetically to the fact that God's intervention will break the betrothal covenant so that Immanuel will be born. Hence the use of the interjection hinneh "behold," which calls attention to the word ha-‘almah "the concealed one," and thus signals a miracle or uncommon event. Symbolically, with the birth of Immanuel, a covenant was broken so that a new covenant (between God and his people) could be initiated.
The Septuagint, in Isaiah 7:14, as in other instances, proves to be rather an interpretation of the Hebrew text, although the reading proposed by the Greek version, he parthenos "the virgin," does not conflict with the Hebrew text, for the meaning "virgin" is implied in the Hebrew term ha-‘almah "the concealed one" (betrothed). As an example of early Jewish interpretation, the Septuagint has its own merit, because it offers new readings and nuances of Hebrew words unknown to today's readers of the Hebrew lexicon, but familiar to Hebrew speakers in the pre-Christian era. A good illustration is the term ‘almah, which designates a marriageable woman, someone who could have been engaged but was not yet married. As we have pointed out in the previous discussion, given the high moral standards regarding marital issues in ancient Israelite society, we may rightly assume that for a fiancé (‘almah) was a common thing to be virgin. Thus the Septuagint's reading (he parthenos "the virgin") is quite plausible on social-ethical and perhaps lexical grounds. Given its use and authority since the time of the apostles, and then throughout patristic period, the Septuagint was seen by Christians as an almost inspired version of the Scripture. Within this pious context, the text of Isaiah 7:14 has been always read as a reference to the virginal conception and birth of Christ. In our opinion, in order to gain a complete profile of the one who conceived and gave birth to Immanuel, we should take a closer look at both venerable biblical versions, namely, the Hebrew text and the Septuagint. The conclusion drawn from a linguistic investigation of Isaiah 7:14 is that Messiah's mother will be a "virgin" (parthenos), and the birth of Messiah will occur during her betrothal period, when she will be "concealed" (from her future husband).
According to Luke, the fulfillment of Isaiah's prophecy is announced by the angel Gabriel to a virgin by the name of Mary, betrothed to a man, Joseph, and living in Nazareth of Galilee at the time of the announcement. Gabriel gives Mary the good news that she will conceive and give birth to a son, who will be named Jesus.  This son will be both the son of the Most High and the son of David whose throne he will occupy (Lk 1:30-33). The dialogue between Mary and Gabriel, according to the evangelist Luke, finishes by pointing to the virginal conception and birth of Jesus, made possible exclusively by God's wonderful power.
"Mary said to the angel, ‘How can this be, since I do not know a man?' The angel said to her, ‘The Holy Spirit will come upon you, and the power of the Most High will overshadow you; therefore the child to be born will be holy; he will be called Son of God'" (Lk 1:34-35).
Mary's reply to the angel's news, "I do not know a man," refers to the absence of any intimate relationship with a man, and thus marks the betrothal period during which time the engaged ones were living in separate homes. It does not imply any "monastic" vow to spend one's life in absolute chastity, and, in fact, ancient Israelite society did not know such an institution.  The brief note on Mary being betrothed to Joseph supports this interpretation. We may note that the conception of Jesus is described in terms of a new creation, and hence the imagery of the Holy Spirit coming upon Mary, which parallels its manifestation in the beginning, as the Spirit of God hovering over the primordial watery deep, keeping a vigilant eye on the possible enemies of the Creator (Gen 1:2).
The title "Most High"  mentioned by Gabriel is a famous name for God used in the Hebrew Bible, which is found in texts dealing with themes such as creation, and royalty.  Majesty, power, universalism, and transcendence are the main characteristics of God Most High, of which Psalm 97:9 may offer a good illustration, "For you are Yahweh, Most High over all the earth; you are exalted far above all gods." In the biblical tradition, this, possibly Canaanite, god ‘el ‘elyon "God Most High" was equated with Yahweh.  The use of the name "Most High" in Luke's narrative of annunciation fits quite well with this understanding, for the angel's emphasis is on God's powerful intervention. Messiah's incarnation is an act of power, and is due exclusively to God.
In summary, while the Hebrew word betulah "virgin" (Greek parthenos) emphasizes the idea of chastity,  the term ‘almah  hints at the fact that the young woman so labeled was independent, living alone or with her parents, yet separated from her fiancé or future husband, in a state of seclusion, with little or no public appearances. In the Hebrew reading of Isa 7:14 and in its New Testament application (Mt 1:18, 22-23; Lk 1:26-27, 30-31), it is not merely the chastity, but rather the seclusion of Messiah's mother during the moment of conception, that is primarily underlined. Although concealed from her fiancé and unknown to the world at large, Mary was nevertheless known by God, and willing to submit herself to his will. And this is the core of our proposed etymology for and rendition of ha-‘almah in Isa 7:14 as "the concealed one."
 The feminine noun ‘almah appears nine times in the entire Hebrew Bible: Gen 24:43 (of Rebekah); Exod 2:8 (of Moses' sister); Ps 68:26 (of Israelite maidens playing tambourines); Song 1:3 (of king's maidens); 6:8 (of maidens, associated with queens and concubines); Prov 30;19 (of a young woman about to be married); Isa 7:14 (of Immanuel's mother); Ps 46:1; 1 Chron 15:20. Except for Ps 46:1 and 1 Chron 15:20, where ‘alamot (plural of ‘almah) may designate a type of (liturgical) singing, in all other instances, the term ‘almah indicates a young, unmarried woman. Note that the masculine form ‘elem (‘alem) "young man, lad" appears in only two texts: 1 Sam 17:56 (of David victorious over Goliath); 20:22 (of Jonathan's servant).
 HALOT stands for Koehler, L., and W. Baumgartner, eds. The Hebrew and Aramaic Lexicon of the Old Testament. 5 vols. 3rd edition. Leiden: Brill, 1994-2000.
 As for the etymology, the editors of HALOT derive the noun ‘almah from a root ‘-l-m III, meaning "to be strong," unattested in Hebrew, but found in Ugaritic glm "to be agitated," Aramaic ‘lm "to be powerful," Arabic galima "to be filled with passionate desire." According to Gesenius' lexicon, this verb is denominative, thus it cannot help much in defining the semantics of the discussed noun.
 The Septuagint renders the Hebrew term as he parthenos "the virgin," but the three second-century Greek translations, Aquila, Symmachus and Theodotion, read neanis "young woman, girl." (Note that Isa 7:14 and Gen 24:43 are the only instances where the Septuagint renders the Hebrew term ‘almah with parthenos [so Vulgate virgo].) The Isaiah Scroll found at Qumran (1QIsaa), and copied in the 2nd century BC, has the same reading as the Masoretic Text, ‘lmh. The Targum of Jonathan on Isaiah reads ‘wlymt' "young woman, girl," while the Syriac Peshitta, perhaps under the influence of the Septuagint, uses a Hebraism, btwlt', meaning "virgin."
 A similar situation obtains with Psalm 22, another messianic text, where Messiah's mother is twice mentioned (vv. 10, 11) in sharp contrast with the absence of any mention of the physical father.
 Note that in the name Immanuel (‘immanu'el) "God [is] with us," the verb "to be" is implied.
 In Greek ho ön. This is the Septuagint reading of the Hebrew name YHWH in Exod 3:14.
 Corresponding to the Septuagint parthenos "virgin." The Hebrew term for "virgin," betulah, means specifically an adult woman without any sexual experience with men. The root *b-t-l from which the Hebrew noun betulah derives, though unattested in Hebrew, is found in Arabic, batala, meaning "to separate"; popular Arabic, "to live in chastity"; hence the idea of chastity; Arabic batul "sacred virgin"; Akkadian batulu "young man" [feminine "virgin"]; Ugaritic btlt, an attribute of goddess ‘Anat. It is important to note that in the Hebrew Bible, ‘almah is used as a parallel term for betulah.
 I.e., the derivation of the noun ‘almah from the root œ-l-m I "to be concealed, hidden." Note the fanciful interpretation of one of the Midrashim pointing to a similar etymology for ‘almah, "And the maiden went" (Exod 2:8). ‘Why does the verse call her ‘almah? Because she set about her errand speedily. Rabbi Samuel said, because she concealed (he-‘elimah) her identity" (Midrash Rabbah Shemoth, 1.25). Note also the Septuagint's rendition of ‘alamot in Ps 46:1 (of uncertain meaning: "[singing] in the style of young girls, soprano") with ton kyphrion "the concealed ones" (from verb krypto "to hide, conceal").
 From the Hebrew yesha', meaning "salvation."
 We may mention that Israelite law had provisions for a similar institution, i.e., the Nazirite vow (Num 6), but this was only a temporal convention, and did not refer at all to sexual abstinence.
 Hebrew ‘el-‘elyon "God Most High"; cf. Greek hypsistos "Most High."
 E.g., Gen 14:18; Num 24:16; Isa 14:14.
 Cf. Gen 14:22; Ps 47:2.
 The Hebrew word betulah, designating a grown-up girl without any sexual experience with men (Gen 24:16), who has no husband (Lev 21:3; Ju 12:12), stands in sharp contrast with widow and repudiated wife (Lev 21:14; Ezek 44:22).
 In Gen 24:16, the ‘almah Rebekah (v. 43) is defined as "virgin (betulah) whom no man has known"; thus the term ‘almah may include the aspect of virginity or chastity underlined primarily yet not exclusively by the word betulah.
 In Exod 2:8, Moses' sister is labeled ha-‘almah (Septuagint: he neanis). In the context of Exod 2, where na'arot "female servants" (v. 5) of Pharaoh's daughter and a meneqet "nurse" (v. 7) from the Israelite women, and the ‘amah "slave" of Pharaoh's daughter (v. 5) are mentioned, this ‘almah can indicate, as in Gen 24 (and 1 Sam 17, with respect to ‘alem, the male counterpart), an independent young woman, still living with her parents.