:: The top-rated definition ::
"back up" is the act of making a copy of files, data or hard drive in case the original is lost or damaged.
Please back up the data every night.

:: Note ::
The file backed up is called as a backup. It is written as one word when it is used as a noun or adjective.
The situation of the vehicles waiting in a long line to continue moving.
It's starting to back up, I may be late for the meeting.
back up
An accumulation in toilets, sink or traffic because of something blocking the flow.

The action of moving backward.
If you back up your car a bit further, I can park too.
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