1611, from Fr. rétroactif (fem. rétroactive) "casting or relating back," from L. retroactus, pp. of retroagere "drive or turn back," from retro- "back" + agere "to drive, set in motion" (see act).
Main Entry: ret·ro·ac·tive Pronunciation: "re-trO-'ak-tiv Function: adjective : extending in scope or effect to a prior time or to conditions that existed or originated in the past; especially: made effective as of a date prior to enactment, promulgation, or imposition retroactive tax> —see also EX POST FACTO LAW —ret·ro·ac·tive·lyadverb —ret·ro·ac·tiv·i·ty/-ak-'ti-v&-tE/noun
Main Entry: ret·ro·ac·tive Pronunciation: "re-trO-'ak-tiv Function: adjective : having relation or reference to or efficacy in a priortime; specifically: relating to, caused by, or being obliteration of the results of learning by immediately subsequent activity <retroactive inhibition>
[re-troh-ak-tiv] 
troʊˈæk