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):

main entry word
    n 1: the form of a word that heads a lexical entry and is
         alphabetized in a dictionary [syn: citation form, main
         entry word, entry word]