| Definition -
A morpheme that can stand on its own as a word. As opposed to a bound morpheme, which is a morpheme that can't stand alone as a word.
For example, the word undone has two morphemes: the bound morpheme un- and the free morpheme done.
Oxford English Dictionary - The term's first citation is from 1933: "Morpheme-words, consisting of a single (free) morpheme: man, boy, cut, run, red, big."
(L. Bloomfield Language xiii. 209)