*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 ]

☞ 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 ]