Around 734 to 732 BCE, with Judah under threat from an alliance of Israel and Aram-Damascus, Isaiah tells King Ahaz that God will give him a sign: a young woman is with child and will bear a son to be called Immanuel, "God with us," and before the boy is old enough to know good from evil, the land of the two kings Ahaz fears will be deserted. Read in its own setting, the sign is dated to Ahaz's own crisis and resolved within a few years of it, not centuries later.
The Hebrew word translated "young woman," almah, denotes a woman of childbearing age and does not itself specify virginity, unlike the more specific Hebrew word betulah. The Greek Septuagint translation, made some centuries before Jesus's birth, rendered almah with parthenos, a word that does carry the sense of virgin, and Matthew's Gospel cites this Greek wording directly, stating that Jesus's virgin birth fulfilled it. Jewish tradition, working from the Hebrew text and the passage's own historical frame, has read the child differently, in various interpretations as Ahaz's own son and successor Hezekiah or simply an unnamed child of Isaiah's own generation, with the sign's force resting on its short-term timing rather than on the child's identity as such.
Sources The Hebrew Bible (Tanakh)Anonymous (multiple authors) with When God Spoke Greek: The Septuagint and the Making of the Christian BibleTimothy Michael Law, The New Testament, Gospel of MatthewAnonymous