(UK) A proposed seamless border between Ireland and Northern Ireland in the event of Brexit.

A thing or a person put in the rear or in the back of something to reinforce, hold, support.

A default arrangement that holds if all else fails.

(baseball) A wall or fence behind home plate.

(baseball, slang) A catcher; the position of catcher.

(rounders) The player who stands immediately behind the striking base.

(espionage) Something serving to bolster or support a cover story etc.

(transitive) To serve as backstop for.

(transitive) To bolster, support.