Back-Up Wife 

A girl who you will become your wife if the two of you are single at a pre-determined age.

Robin: Let's make a pact, if we both turn 40 and we're both single..
Ted: Robin Scherbatsky, will you be my Back-Up Wife?
Robin: A girl always dreams of hearing those words. Yes, yes, a million times, yes!

Back-Up Wife by drewdaspot June 6, 2012

backup wife 

A backup wife is a wife picked out by a current husband and wife couple for when the current wife dies.

It’s so sad Jane passed away at 40, but at least she had already picked Sue out as a backup wife for John and the kids.

backup wife by EuniceFrasierr June 30, 2020