Retroactive has been around since the 17th century and designates operation with respect to something in the past, such as a retroactive law. The Latin word retroagere, an ancestor of the adjective retroactive, means "drive or turn back," and goes along with the meaning of the word.
This means that RETROACTIVE extends in scope or effect to a prior time or to conditions that existed or originated in the past.
Sometimes governments pass rulings that are set as if they were in effect before the ruling was even made, and that means they are retroactive.
You might be awarded a salary raise that is retroactive, meaning you'll get paid more for work you did in the past. And, retroactive fads in clothing keep vintage clothing stores in business.