lexical base term
Lexical base terms are words or list of words, which form a lexical base for a number of keywords or words, which have got the same meaning. There is no clear definition what the lexical base term for a keyword might be, since this depends much from the context of the application.
'maintain' could be the lexical base for
maintain, maintained, maintains, maintenance
'maintenance' could also refer to a different lexical base term
maintenance