Backstop

A backstop, by the way, is used in baseball - it's a fence or screen behind the catcher, designed to stop the ball leaving the area if it's not hit or caught. So, for the purposes of Brexit, think of something along the lines of a safety net.

In the news

 * Donald Tusk @Twitter, 20 August 2019: The backstop is an insurance to avoid a hard border on the island of Ireland unless and until an alternative is found.