back·up 
/ˈbækˌʌp/ noun
plural back·ups /ˈbækˌʌp/ noun 1 a [count] : a person or thing that can be used to replace or support another person or thing
▪ His role on the team is to be a backup to the regular quarterback. ▪ We have an extra radio as a backup in case this one doesn't work. — often used before another noun ▪ a backup quarterback ▪ We need a backup plan in case of emergency.
b [noncount] : help or support provided by additional people or things
▪ The policeman called for backup [=called for other police to come and help him] because the suspect was carrying a gun. ▪ We have an extra radio for backup. ▪ He provides backup for the regular quarterback. 2 [count] US : a situation in which the flow of something (such as traffic) becomes blocked
▪ There was a traffic backup [=(Brit) tailback] for miles on the road because of the accident. ▪ There was a backup [=blockage] in the drain. 3 [noncount] : a musical part that is sung to support the main singer
▪ She sang backup on his CD. — often used before another noun ▪ a backup singer 4 [count] : a copy of information stored on a computer
▪ Be sure to make a backup of your work. — often used before another noun ▪ a backup file/copy ; also : the act or process of making such a copy
▪ It may take some time for the backup to be complete. — see also back up at 4back
▪ His role on the team is to be a backup to the regular quarterback. ▪ We have an extra radio as a backup in case this one doesn't work. — often used before another noun ▪ a backup quarterback ▪ We need a backup plan in case of emergency.
b [noncount] : help or support provided by additional people or things
▪ The policeman called for backup [=called for other police to come and help him] because the suspect was carrying a gun. ▪ We have an extra radio for backup. ▪ He provides backup for the regular quarterback.
▪ There was a traffic backup [=(Brit) tailback] for miles on the road because of the accident. ▪ There was a backup [=blockage] in the drain.
▪ She sang backup on his CD. — often used before another noun ▪ a backup singer
▪ Be sure to make a backup of your work. — often used before another noun ▪ a backup file/copy ; also : the act or process of making such a copy
▪ It may take some time for the backup to be complete. — see also back up at 4back
