Because the translation of almah in Isaiah has been such a theologically loaded issue over the centuries, it is worth pursuing it a bit further � which I might begin doing by observing that, while I am no Greek scholar, my Liddell and Scott's Greek-English Lexicon, which has served many generations of Greek students as a standard reference work, translates parthenos as "a maid, maiden, virgin," and lists its adjectival meanings as "virgin, pure, chaste, unsullied." Moreover, my Unabridged Oxford English Dictionary defines both "maid" and "maiden" as "1. A girl; a young (unmarried) woman; 2. A virgin."
It is certainly the case that there is an ambiguity in all these definitions, since even in sexually strict societies not every young unmarried woman is a virgin. Yet it is also the case that, had the translators of the Septuagint wished to be less ambiguous, they might have chosen words other than parthenos for Isaiah's almah, such as kor�, which can mean either "young woman" or "young wife" with no implication of virginity at all; pais, or neanis, which also means a young woman and is the Septuagint's word for Ruth the Moabite when Boaz asks of her: "Who is this damsel?" (The Hebrew word for "damsel" here is na'arah � which, as Mr. Deutsch correctly says, is translated by the Septuagint in the story of Dina as parthenos. But Mr. Deutsch is wrong about parthenos designating Dina after her rape, since it in fact describes her beforehand. Nor do I know what makes Mr. Siegel think that parthenos normally would be used in ancient Greek for a young mother.)
I don't have time to do a word study of the Septuagint use of parthenos right now, but my sense is that Philologos is right. The basic meaning of the word a young, unmarried woman, implicitly a virgin. It should also be noted that the translation of the Greek version of Isaiah is a rather free paraphrase and the translator seems not have known Hebrew very well.