back up

Definitions

noun a copy of a file or set of data kept for security against errors in the original or master copy

verb (of the non-striker) to begin to move down the pitch as the bowler delivers the ball in order to maximise the chances of taking a run should the striker hit the ball
Citation ‘Evans was nearly caught wide on the leg side by Langley … then poor Bailey was run out backing up to Evans’ (Cardus 1978)
verb (of a fielder) to move into a position behind another fielder in order to cover him in case he misses the ball; the term is applied especially to a fielder covering either the wicket-keeper or the bowler at the non-striker’s end when a ball is thrown in from the outfield
Citation ‘The man who ought to be abused when an overthrow occurs is not the fieldsman who throws the ball but the men who should be backing up and are not’ (Badminton 1888)

(written as back-up)
To copy computer files onto an additional storage medium. For instance, copying the contents of a folder contained on a hard drive to a tape drive or floppy disks. This is important, for instance, in avoiding the loss of data when there are software or hardware malfunctions, or in case of user carelessness. There are utilities which can simplify this process, for instance, by following a schedule, or by compressing the data to economize storage space and time.

verb to make a copy of computer data to keep in case anything goes wrong with the original

noun
(written as back-up)
the accompaniment to the main performer of a piece of popular music or jazz
verb to print on the second side of a sheet of paper

noun
(written as back-up)
(someone who is) prepared to use force on behalf of or otherwise show solidarity with (a friend). The term, deriving from the colloquial verb phrase ‘back (someone) up’, was first part of the vocabulary of gangs, and since around 2000 extended to other speakers.