The answer is:
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Retro- () — A prefix or combining form signifying backward, back; as, retroact, to act backward; retrospect, a looking back.
Retroact (v. i.) — To act backward, or in return; to act in opposition; to be retrospective.
Retroaction (n.) — Action returned, or action backward.
Retroaction (n.) — Operation on something past or preceding.
Retroactive (a.) — Fitted or designed to retroact; operating by returned action; affecting what is past; retrospective.