Source : Webster's Revised Unabridged Dictionary (1913)

Lemma \Lem"ma\ (l[e^]m"m[.a]), n.; pl. L. {Lemmata}
   (-m[.a]*t[.a]), E. {Lemmas} (-m[.a]z). [L. lemma, Gr. lh^mma
   anything received, an assumption or premise taken for
   granted, fr. lamba`nein to take, assume. Cf. {Syllable}.]
   A preliminary or auxiliary proposition demonstrated or
   accepted for immediate use in the demonstration of some other
   proposition, as in mathematics or logic.

Source : WordNet®

lemma
     n 1: a subsidiary proposition that is assumed to be true in order
          to prove another proposition
     2: the lower and stouter of the two glumes immediately
        enclosing the floret in most Gramineae [syn: {flowering
        glume}]
     3: the head of an annotation or gloss
     [also: {lemmata} (pl)]

lemmata
     See {lemma}