Lexeme

Lexeme is a term in linguistics. They are units of meaning, independent of any inflectional endings, or whether it is one word or several. "Come in" is a lexeme; so is "raining cats and dogs". The largest English dictionaries have about half a million lexemes. "The true figure is undoubtedly a great deal higher". 
In the English language, "run", "runs", "ran" and "running" are forms of the same lexeme, conventionally written as RUN.
A similar concept is the lemma (or citation form). It is the form of a lexeme which is chosen to represent the lexeme, for example, RUN instead of the other forms. Lemmas are used in dictionaries as the headwords. Other forms of a lexeme are usually listed later in the entry. So all headwords in a dictionary are lexemes.
A "lexicon" is made of lexemes.