Definition from Nolo’s Plain-English Law Dictionary

A law or court decision that takes away or impairs a previously vested right, imposes new duties or obligations, or changes or effects past transactions or legal actions. Retroactive (or retrospective) laws are not favored and, unless it is expressly stated, it is usually presumed that legislation is not intended to apply retroactively. In criminal law, statutes which would increase penalties or make activities which had been previously legal criminal are prohibited by the Constitutional ban on ex post facto laws.

Definition provided by Nolo’s Plain-English Law Dictionary.