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.
(..) - Myumi. How are you? I'm Bob. - Oh! Bob. Bob from United States, man. Ahh! Hi, Bob. Maybe you'd like to sing backup for me. - This is my surfing teacher. - He's your surfing teacher? Sometime, sometime. You're a surfer? Oh, oui, oui. The emperor's house? Cool. Yeah? Yeah. Cool! Right. (..)2003 Lost in Translation