| Definition - (Aka lexeme | lexical item)
A set of words that has the same base form.
For example, run, runs, ran and running are all members of the same lexical unit; the base form is run.
Oxford English Dictionary - The term's first citation is from 1951:
"In the Oxford English Dictionary there are nearly half a million lexical units."
(G. A. Miller Lang. & Communication iv. 89)