1. （决定或行为）有追溯效力的
If a decision or action is retroactive, it is intended to take effect from a date in the past.

1. affecting things past

e.g. retroactive tax increase
an ex-post-facto law
retro pay

Synonym: ex post factoretro

2. descriptive of any event or stimulus or process that has an effect on the effects of events or stimuli or process that occurred previously