word-forming element meaning "not, opposite of, without" (also im-, il-, ir- by assimilation of -n- with following consonant, a tendency which began in later Latin), from Latin in- "not," cognate with Greek an-, Old English un-, all from PIE root *ne- "not."
In Old French and Middle English often en-, but most of these forms have not survived in Modern English, and the few that do (enemy, for instance) no longer are felt as negative. The rule of thumb in English has been to use in- with obviously Latin elements, un- with native or nativized ones.
late 14c., deffinen, diffinen, "to specify; to fix or establish authoritatively;" of words, phrases, etc., "state the signification of, explain what is meant by, describe in detail," from Old French defenir, definir "to finish, conclude, come to an end; bring to an end; define, determine with precision," and directly from Medieval Latin diffinire, definire, from Latin definire "to limit, determine, explain," from de "completely" (see de-) + finire "to bound, limit," from finis "boundary, end" (see finish (v.)). From c. 1400 as "determine, declare, or mark the limit of." Related: Defined; defining.