Definition of "backstop"

Irish prime minister Leo Varadkar, this morning, said a time-limited backstop would be unacceptable, and has previously promised to vote down the UK’s Brexit withdrawal deal unless it features a satisfactory backstop.

The Express, 7 June 2018

"The 2½ years since have been about hammering home these points in any European capital that would listen. And in spite of capacity-sapping talks on the first point – Brexit, border and backstop – the Department of Foreign Affairs and other government departments have been working quietly to make good on the second.

The Irish Times, 17 November 2018

Bench may be the greatest catcher subjectively, but he's not statistically. Statwise, Yogi Berra is the all-time backstop, regardless of which measure you prefer. Berra's Total-Z is one-third of a standard deviation higher than Bench's, and his Core-Z average is a point better.

2002, James F. Vail, Outrageous Fortune: What's Wrong with Hall of Fame Voting and How to Make It Statistically Sound, Jefferson, NC: McFarland & Company, page 143

[…] to be set up in Indonesia with a phony book and school supplies company established in New York City as a backstop.

1976, Joseph Burkholder Smith, Portrait of a Cold Warrior, page 116

Closely representing an alibi, creating a backstop could involve such measures as having people available to verify the details of an agent's cover story.

1990, Bruce W. Watson, Susan M. Watson, Gerald W. Hopple, United States intelligence: an encyclopedia, page 1974

Russia signalled on Monday it would backstop the European Union's bailout of Cyprus despite anger that the weekend rescue deal would impose heavy losses on uninsured depositors, many of them Russian.

2013 March 26, Douglas Busvine, Darya Korsunskaya, “Russia backstops Cyprus bailout despite anger”, in Reuters‎