From BiblePortal Wikipedia

Watson's Biblical & Theological Dictionary [1]

עלמה , a Hebrew word signifying properly, a virgin, a young woman, unacquainted with man. In this sense it occurs in the famous passage of   Isaiah 7:14 : "Behold a virgin shall conceive and bear a son." The Hebrew has no term that more properly signifies a virgin than almah. St. Jerom, in his commentary on this passage, observes, that the Prophet declined using the word bethaul which signifies any young woman, or young person, but employed the term almah, which denotes a virgin never seen by man. This is the import of the word almah, which is derived from a root which signifies to conceal. It is very well known, that young women in the east do not appear in public, but are shut up in their houses, and their mothers' apartments, like nuns. The Chaldee paraphrast and the Septuagint translate almah "a virgin;" and Akiba, the famous rabbin, who was a great enemy to Christ and Christians, and lived in the second century, understands it in the same manner. The Apostles and Evangelists, and the Jews of our Saviour's time, explained it in the same sense, and expected a Messiah born of a virgin.

The Jews, that they may obscure this plain text, and weaken this proof of the truth of the Christian religion, pretend that the Hebrew word signifies a young woman, and not a virgin. But this corrupt translation is easily confuted.

1. Because this word constantly denotes a virgin in all other passages of Scripture in which it is used.

2. From the intent of the passage, which was to confirm their faith by a strange and wonderful sign. It surely could be no wonder, that a young woman should conceive a child; but it was a very extraordinary circumstance that a virgin should conceive and bear a son.

Webster's Dictionary [2]

(n.) Same as Alme.