The Collaborative International Dictionary
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.
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.
n. (context legal English) Any law that makes a prior legal act illegal, or increases the penalty for a prior illegal act.