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.
(..) This isn't gonna be like New York, pal. This suit's already kicked the crap outta the Hulk. Guys! Vision needs backup, now! Hulk? Hulk, I know you like making your entrance at the last second. Well, this is it, man. This is the last, last second. (..)2018 Avengers: Infinity War
(..) life, I don't work for you. And due to that fact, we're now in a flying doughnut... billions of miles from Earth with no backup. - I'm backup. - No, you're a stowaway. The adults are talking. I'm sorry, I'm confused as to the relationship here. (..)2018 Avengers: Infinity War