LONDON — After months of tortured debate over Britain’s departure from the European Union, a new word has entered the jargon-strewn Brexit lexicon: “flextension.” It could hold the key to the next phase.

Flextension is how insiders are describing the type of delay to Brexit that Prime Minister Theresa May now seems likely to seek from the bloc: a long extension that could be cut short if Parliament finally approves an exit deal.

If it sounds dubious, it is nevertheless considered an improvement on an earlier term for the process, “terminability.”

Whatever it is called, analysts are far from sure that Mrs. May will succeed in selling the idea when she meets European Union leaders at a summit in Brussels next Wednesday. And any one of them could veto any extension, leaving Britain facing a disorderly Brexit without an agreement on April 12, a deadline that was already pushed back once.