Fait accompli /ˈdībak/ noun 

Definition: Fait accompli is a noun that means an irreversible accomplishment. It often refers to a change or decision made by some authority on behalf of the people who will be affected. For example, if workers continue to strike after their working conditions change, they’re protesting a fait accompli.

Etymology: It comes from French and literally means “accomplished fact.” 

In a Sentence

The workers continued to strike even though it was a fait accompli because the company had already made changes.

She dumped him after he cheated on her. Even though he knew it was a fait accompli, he continued trying to win her back.

He wished he could change his boss’s mind, but he knew his boss’s decision was a fait accompli.

Synonyms 

Certainty, Done deal, Done deed

Antonyms 

Temporary Solution, Group Decision, Democratic Process