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.
"producing an effect; having power to produce an effect," late 14c., Old French effectuel, from Late Latin effectualis, from Latin effectus "accomplishment, performance" (see effect (n.)). Used properly of actions (not agents) and with a sense "having the effect aimed at" (effective, by contrast, is used of the agent or the thing done and with a sense "having great effect"). Related: Effectually; effectualness.