The prophecy in Isaiah 7:14 came up on the Matthew thread. Over Christmas, I came across
this video by two Israeli Christians discussing whether or not Isaiah was referring just to a young woman or to a virgin. They used Isaiah 54:4-5 as evidence that the word almah means an adolescent woman, who is unmarried, as implied by the parallelism in verses 4-5, which I've indicated using green and blue highlights:
1Sing, O barren one, who did not bear;
break forth into singing and cry aloud,
you who have not been in labor!
For the children of the desolate one will be more
than the children of her who is married,” says the Lord.
2“Enlarge the place of your tent,
and let the curtains of your habitations be stretched out;
do not hold back; lengthen your cords
and strengthen your stakes.
3For you will spread abroad to the right and to the left,
and your offspring will possess the nations
and will people the desolate cities.
4Fear not, for you will not be ashamed;
be not confounded, for you will not be disgraced;
for you will forget the shame of your youth (alumayik),
and the reproach of your widowhood (almenutayik) you will remember no more.
5
For your Maker is your husband,
the Lord of hosts is his name;
and the Holy One of Israel is your Redeemer,
the God of the whole earth he is called.
The shame of her youth is remedied by God becoming her husband, and the reproach of her widowhood is remedied by God becoming her redeemer. These have in common the lack of a husband. This could indicate that the word almah carries the meaning of being an unmarried young woman, and by implication given the Hebrew culture, a virgin.
As pointed out in the comments, verse 1 refers to a woman who is sterile, not an unmarried woman. Does this influence the meaning of verses 4 and 5? The thought seems to jump from sterility (v1) to lack of a husband (v4-5)?