Source : Webster's Revised Unabridged Dictionary (1913)

Retroactive \Re`tro*act"ive\, a. [Cf. F. r['e]troactif.]
   Fitted or designed to retroact; operating by returned action;
   affecting what is past; retrospective. --Beddoes.

   {Retroactive law} or {statute} (Law), one which operates to
      make criminal or punishable, or in any way expressly to
      affect, acts done prior to the passing of the law.

Source : WordNet®

retroactive
     adj 1: descriptive of any event or stimulus or process that has an
            effect on the effects of events or stimuli or process
            that occurred previously [ant: {proactive}]
     2: affecting things past; "retroactive tax increase"; "an
        ex-post-facto law"; "retro pay" [syn: {ex post facto}, {retro}]