retroactivity

adjective | \ ˌre-trō-ˈak-tiv \ | ret·ro·ac·tive

1. : extending in scope or effect to a prior time or to conditions that existed or originated in the past; especially : made effective as of a date prior to enactment, promulgation, or imposition a retroactive tax — see also ex post facto law : made effective as of a date prior to enactment, promulgation, or imposition a retroactive tax — see also ex post facto law