Jen McCown explains the different between backup and back up:
Let it be known that the word “backup” is a noun (it refers to a thing), and “back up” is a verb (it refers to an action.
I’m going to back up the database.
It will produce a backup.
I’ll save that backup until we back up the database three more times.
Now back up a moment here; I need to come up with a backup plan…