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.
(..) - Hey!! - Gerry. Multiple -s at Washington Park. Please, send units. We're falling back on Reed Street. Two Robert , units need backup. Mobile ops is down. Please respond. Two Robert , are you there? Copy. - Girls!! - Responders, Code , Code . Rachel, quick, come on. (..)2013 World War Z