What Is the Irish Backstop, and Why Is It Holding Up Brexit?

LONDON — The Irish backstop — convenient shorthand for a devilishly complex subject. It sounds almost like a fence, but the issue is about not having a fence at all — an apt paradox for a problem that may not have a solution.

And it has become one of the overriding sticking points in Brexit, Britain’s halting, seemingly interminable effort to leave the European Union.

Government officials debate it daily, in London and on the Continent, and ordinary Britons and Irish people are discussing it, too, if only to say that they don’t understand what it means.