IPA: /ˌrɛtrəʊˈæktɪvli/
KK: /ˌrɛtroʊˈæktɪvli/
Done after something has happened, affecting things that occurred earlier.
The new law was applied retroactively, changing the outcomes of previous cases.
Retroactively → It is formed from "retro-" (meaning backward) and "act" (from Latin "agere", meaning to do or to act) and the suffix "-ively" (meaning in a manner). The word "retroactively" means acting backward in time, often referring to applying a law or decision to a time before it was enacted.
Think of 'backward' ('retro-') actions ('act') that apply to a previous time — that's what retroactively means.