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]

The Collaborative International Dictionary of English v.0.48:

headword \headword\ n.
   1. a word that is qualified by a modifier.
      [WordNet 1.5]

   2. a word placed at the beginning of a line, paragraph, or
      short article (as in a dictionary or encyclopedia entry);
      the word which forms the title of an entry in a
      dictionary.
      [WordNet 1.5]

WordNet (r) 3.0 (2006):

headword
    n 1: a content word that can be qualified by a modifier [syn:
         headword, head word]
    2: a word placed at the beginning of a line or paragraph (as in
       a dictionary entry)