[...] we shall introduce another term, "lexeme", to denote the more "abstract" unit which occurr in different inflexional "forms" according to the syntactic rules involved in the generation of sentences. Notationally, lexemes will be distinguished from words by the use of capitals. Thus the ortographic word "cut" represents three different inflexional "forms" (i.e. three different grammatical words) of the lexeme CUT. [...] Lexemes (the "words" of traditional grammar) are the underlying invariant units considered in abstraction from their "accidental" properties: lexemes are "substances" which occurr in various "accidental" "forms".
- Lyons (2004), a pag.17