Descriptive of any event or stimulus or process that has an effect on the effects of events or stimuli or process that occurred previously.

Roget 179: counteracting etc. v.; antagonistic, conflicting, retroactive, renitent, reactionary; contrary etc. 14.   

Roget 277: recoiling etc. v.; refluent^†, repercussive, recalcitrant, reactionary; retroactive.   

Roget 122: past, gone, gone by, over, passed away, bygone, foregone; elapsed, lapsed, preterlapsed^†, expired, no more, run out, blown over, has-been, that has been, extinct, antediluvian, antebellum, never to return, gone with the wind, exploded, forgotten, irrecoverable; obsolete etc. (old) 124.    former, pristine, quondam, ci-devant [Fr.], late; ancestral.    foregoing; last, latter; recent, over night; preterperfect^†, preterpluperfect^†.    looking back etc. v.; retrospective, retroactive; archaeological etc. n.. ... show more

Affecting things past:
— Retroactive tax increase.

synonyms: ex post facto, retro.