Canon and retcon
Also said as Canon, Retcon, Retroactive continuity
Canon is what officially counts as having happened in the world. A retcon changes that after the fact, so something established is now said to have been different.
Every fact you publish is a thing later work must agree with. A retcon can save a series, but its cost is everything that was built on the old fact, and fans keep count.
Say it
“Check what depends on this before we retcon it. If the king was never poisoned, three quests and the whole of Act 2 need new reasons, so I would rather add a second poisoner.”
Where you’ve seen it
The Elder Scrolls II: Daggerfall has several endings, so the series later decided they all happened: a "Dragon Break" called the Warp in the West let time itself split, turning a problem of canon into part of the lore.
Not to be confused with plot hole
A plot hole is an accident that nobody has addressed. A retcon is a deliberate change that says: this is how it was all along.