The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]

   Retroactive law or Retroactive 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.
      [1913 Webster]

WordNet (r) 3.0 (2006):

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,
       retroactive, retro]

Moby Thesaurus II by Grady Ward, 1.0:

24 Moby Thesaurus words for "retroactive":
   a priori, antiphonal, back, backward, early, ex post facto,
   into the past, reacting, reactionary, reactive, recessive, reflex,
   reflexive, refluent, regressive, respondent, responding,
   responsive, retroactionary, retrocessive, retrograde,
   retrogressive, retrospective, revulsive