The Collaborative International Dictionary of English v.0.48:

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.]
   1. (Math., Logic) A preliminary or auxiliary proposition
      demonstrated or accepted for immediate use in the
      demonstration of some other proposition, as in mathematics
      or logic.
      [1913 Webster]

   2. A word that is included in a glossary or list of
      headwords; a headword.
      [PJC]