What does fait accompli mean?

In French, fait is a conjugated form of the verb faire, which means to do, but it also means fact, which is the applicable context in this phrase. Fait accompli literally translates into fact accomplished.

In English, (and in French) fait accompli means a course of events that has already been completed and cannot be undone, or something that has been decided and is, therefore, a done deal. If, for example, you are the Chairman of an organization and you have the authority to appoint someone to an office, and you have done so, the appointment is not open to discussion, it is a fait accompli.

More recently, fait accompli has been used in a different sense. It is commonly (and incorrectly) used to describe a future task, or a task underway not yet completed, with a very high degree of confidence in the success of the outcome. For example, a prospective home buyer may tell the existing seller of the home that, while he has not yet obtained a mortgage, the financial arrangements are a fait accompli, meaning that the homebuyer is certain there will be no obstacles in securing the financing.
An action that when completed can not be reversed by anyone affected by the action.