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.
(..) an appointment... OFFICER : Batman, put down your weapons and surrender. You're surrounded. - What are you waiting for? - Backup. - Backup? - The Batman's in there. SWAT's on the way. But if you wanna go in now... ...I'll be right behind you, sir. (..)2005 Batman Begins
(..) Rumor was they tested dispersing water-based chemicals into the air... ...but isn't that illegal? I want all the information on the development of this project... ...all data, files, backup disks, on my desk right away. Did you lose one? I'm merging your department with Archives. (..)2005 Batman Begins