| Definition - (Aka lexeme | lexical unit)
A set of words that has the same base form.
For example, run, runs, ran and running are all members of the same lexical item; the base form is run.
Oxford English Dictionary - The term's first citation is from 1964:
"Archaic features may be preserved in different lexical items in different places."
(R. A. Hall Introd. Ling. liii. 254)