To “back up” is an activity; “back up your computer regularly”; “back up the truck to the garden plot and unload the compost.”
A “backup” is a thing: “keep your backup copies in a safe place.” Other examples: a traffic backup, sewage backup, backup plan, backup forces.
Older writers often hyphenated this latter form (“back-up”), but this is now rare.
BUY THE BOOK!
Copy and paste this URL into your WordPress site to embed
Copy and paste this code into your site to embed