An accomplished fact; an action which is completed before those affected by it are in a position to query or reverse it.
The literal translation into English of this French phrase is a fact realized or accomplished - what might these days be called a done deal. Strangely, it entered the English language via a travelogue of Spain rather than France. Richard Ford's A hand-book for travellers in Spain, 1845, was and still is, regarded as a classic of travel writing. In it Ford included the phrase "This is now a fait accompli.", in regard of some previously decided fact.
In 1858, Sir William Stirling Maxwell wrote that "So great a literary achievement had never before been performed under so unpretending an appellation... it took its place among the best books of travel, humour, and history, social, literary, political, and artistic, in the English language".
See also - other French phrases in English.