RETROACTIVE

[ret.ro.ac.tive] adj [F retroactif, fr. L retroactus, pp. of retroagere to drive back, reverse, fr. retro- + agere to drive--more at agent] (1611): extending in scope or effect to a prior time or to conditions that existed or originated in the past; esp: made effective as of a date prior to enactment, promulgation, or imposition "~ tax" -- ret.ro.ac.tive.ly adv -- ret.ro.ac.tiv.i.ty n