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]

The Collaborative International Dictionary of English v.0.48:

main entry word \main entry word\ n.
   The form of a word that heads a lexical entry and is
   alphabetized in a dictionary; also called entry word,
   headword, and lemma.

   Syn: citation form, entry word, headword, lemma.
        [WordNet 1.5]

   Note: In different languages, different wordforms, such as
         cases for verbs, may be taken as the main entry word.
         In English dictionaries, it is the infinitive form, but
         in latin dictionaries it is usually the first person
         singular present.
         [PJC]

WordNet (r) 3.0 (2006):

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: lemma, flowering
       glume]
    3: the heading that indicates the subject of an annotation or a
       literary composition or a dictionary entry

The Free On-line Dictionary of Computing (30 December 2018):

lemma

    A result already proved, which is needed in the proof
   of some further result.

   (1995-03-25)