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!
When someone of either gender, suddenly in a short period of time, start dating another person. Then all of a sudden that's the only thing that matters. Or getting serious in a realtionship too quick.