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.

A wall or fence behind home plate.

A catcher; the position of catcher.

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

The longstop.

The wicket-keeper.

Something serving to bolster or support a cover story etc.

To serve as backstop for.

To bolster, support.