Other forms: backups
A backup is a second copy of your files, kept somewhere else in case the first one is lost. The photos copied onto a hard drive; the phone that saves itself to the cloud while you sleep.
The other backup is people: an officer calling for backup wants more officers, now. As an adjective it means kept in reserve — a backup plan, a backup generator, a backup singer standing behind the star. Mind the space, because this is the mistake everyone makes: back up (two words) is the verb, 'back up your work before you leave', and backup (one word) is the thing you end up with. Americans also speak of a backup of traffic, cars stacked behind a jam, and a backup in a drain when the water refuses to go down.
1nounbackupA spare copy of files or data, kept in case the original is lost.~55%
“The whole system was restored from last night's backup.”
2nounextraExtra people or resources ready to help if they are needed.~15%
“The officer radioed for backup and waited by the car.”
3adjectivefirstKept ready in case the first thing fails.~30%
“We had a backup plan in case it rained.”
Shares are an estimate: a language model read 21 sentences found in the wild and placed 20 of them on a sense (1 were unclear) — a count of our sample, not a measurement of English.
Deverbal from back up.
Etymology from Wiktionary, CC BY-SA