Definition of "back up" in English

Verb

To move backwards, especially for a vehicle to do so.
- That beeping sound indicates that the truck is backing up.
To move a vehicle backwards.
- Back up the car a little, you're blocking the driveway.
To undo one's actions.
- I couldn't see how to finish the project, so I backed up and tried it another way.
To reconsider one's thoughts.
- This isn't working. Let's back up and think about it.
To copy (data) so that it can be restored if the main copy is lost.
- Back up your documents folder before applying the update.
To provide support or the promise of support to.
- You should be careful. This guy is backed up by the local gang.
- When he said I wasn't there, I told him I was, and my buddy backed me up.
For the non-striker to take a few steps down the pitch, in preparation to taking a run, just as the bowler bowls the ball.
For a fielder to position himself behind the wicket (relative to a team-mate who is throwing the ball at the wicket) so as to stop the ball, and prevent overthrows.
To halt the flow or movement of something.
- When I flushed the toilet, the plumbing backed up and burst.
To fill up because of a backlog.
To run a printing press in reverse.