means In mathematics and logic, a lemma is a minor proven proposition that isn't important on its own but serves as a stepping-stone toward proving a larger theorem; in linguistics, it's the dictionary headword form of a word.

from From Greek lēmma, "something taken or received," from the verb lambanein, "to take" — so a lemma is literally a thing you've grasped and now hold in hand. The same root branches into dilemma, a situation where you've "taken twice" and are caught between two options. The plural, fittingly, often keeps its Greek dress: lemmata.