The Hebrew word in Isaiah 7:14 is "almah," and its inherent meaning is "young woman." "Almah" can mean "virgin," as young unmarried women in ancient Hebrew culture were assumed to be virgins. Again, though, the word does not necessarily imply virginity.
"Almah" occurs seven times in the Hebrew Scriptures (Genesis 24:43; Exodus 2:8; Psalm 68:25; Proverbs 30:19; Song of Solomon 1:3; 6:8; Isaiah 7:14). None of these instances demands the meaning "virgin," but neither do they deny the possible meaning of "virgin." There is no conclusive argument for "almah" in Isaiah 7:14 being either "young woman" or "virgin."
However, it is interesting to note, that in the 3rd century B.C., when a panel of Hebrew scholars and Jewish rabbis began the process of translating the Hebrew Scriptures into Greek, they used the specific Greek word for virgin, "parthenos," not the more generic Greek word for "young woman."
The Septuagint translators, 200+ years before the birth of Christ, and with no inherent belief in a "virgin birth," translated "almah" in Isaiah 7:14 as "virgin," not "young woman." This gives evidence that "virgin" is a possible, even likely, meaning of the term.