backstop

Short definition from Wiktionary

Noun

1. 1
   An object or a person put in the rear or in the back of something to reinforce, hold, support.
2. 2
   A default arrangement that holds if all else fails.
3. 3
   A wall or fence behind home plate.
4. 4
   A catcher; the position of catcher.
5. 5
   The player who stands immediately behind the striking base.
6. 6
   The longstop.
7. 7
   The wicket-keeper.
8. 8
   Something serving to bolster or support a cover story etc.

Verb

1. 1
   To serve as backstop for.
2. 2
   To bolster, support.

Proper noun

1. 1
   A proposed seamless border between Ireland and Northern Ireland in the event of Brexit.