Abarim Publications' online Biblical Hebrew Dictionary
Scholars have identified three different roots of the form עלם ('lm) in the Bible. They are formally not related but a poetic audience may think otherwise:
The verb עלם ('alam), meaning to be hidden or concealed (Psalm 90:8, 1 Kings 10:3, Nahum 3:11). This root comes with the derivation תעלמה (ta'alumma), meaning hidden thing (Job 11:6).
The unused and assumed root עלם ('lm II), which yields the masculine derivation עלם ('elem), meaning a [male] youth. This word is surprisingly rare in the Old Testament: it occurs a mere three or four times — in 1 Samuel 16:12, 17:42 and 17:56, it describes David; in 20:22 it describes David's arrow runner — which demonstrates that this word doesn't simply emphasize one's obvious youngness but perhaps rather one's potential, which is a quality that's invisible for all but a few insightful observers.
The female equivalent of this noun, עלמה ('alma), a [female] youth, occurs slightly more often, but still not often enough to describe mere youth (the more common word for young woman is בתולה, betula) and thus probably rather emphasizes a girl's potential to become an entire nation (or two; Genesis 25:23).
This word occurs about a dozen times in the Old Testament, most notably to describe Rebekah (Genesis 24:43), the "maidens" related to templar worship and Solomon's court (Song of Solomon 1:3) and of course the "Virgin" to be with child as mentioned by Isaiah (7:14). The celebrated Greek word that translated this Hebrew original is παρθενος (parthenos), which was an epithet of Pallas Athena; see our article on Mary for more on this.
The third derivative of our root is the noun עלומים ('alumim), which means youth or youthfulness (i.e. potential) in general. It too occurs a mere four times: Job 20:11 and 33:25, Psalm 89:45 and Isaiah 54:4.
The unused and assumed root עלם ('lm III), which gives us the word עולם (olam), meaning forever, ever, everlasting. This word occurs more than three hundred times in the Old Testament, and, strikingly, in twenty instances describes a very long time into the past. This word also demonstrates that the previous words speak of lasting forever and particularly perpetuity while engulfed by things that fleet like grass.