The policeman called for 'backup' [=called for other police to come and help him] because the suspect was carrying a gun.
There was a traffic 'backup' [=('Brit') 'tailback'] for miles on the road because of the accident.
His role on the team is to be a 'backup' to the regular quarterback.
(..) Oh, snap. So, I guess we'll be leaving now. Why should we leave? For all we know he crashed the party and Penny doesn't even want him here. You have a backup hypothesis? Maybe they want to be friends. Or maybe she wants to be friends and he wants something more. Then he and I are on equal ground. (..)2007 TvShow: The Big Bang Theory Title: The Middle Earth Paradigm Season: 1 Episode 6