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.
(..) they will this badge and this division, - Now, do you want to be part of this force? - Yes, sir, All right! Your job won't make you soft, it will harden you to the facts of our time, And when you're on backup, you back up, sir! Yes, sir, So, Mr, Johnson, you still wanna be a peace officer? (..)1994 The Glass Shield