Retcon

A retcon (retroactive continuity) is, per the standard definition, a literary device whereby facts and events established in a narrative are later adjusted, ignored, supplemented or contradicted by a subsequently published work. The classic forms: addition (backstory that fills gaps without contradicting), alteration (new facts that contradict, such as resurrecting the dead) and subtraction (quietly ignoring a story that happened).

The defining trait is deliberateness, which separates a retcon from a plot hole. The term's coined-in-fandom history runs through comics, where the practice earned its name. For serial writers, the useful discipline is the changelog: when canon changes on purpose, write down what changed and why. See what is canon.