Backup vs. Back Up
“Backup” is commonly a noun or adjective for support, a reserve, or an extra copy. “Back up” is the phrasal verb: you back up files, back up a claim, back up a vehicle, or traffic can back up.
Use “backup” for the thing: “Keep a backup.” Use “back up” for the action: “Back up your files.”
Key details
Choose the right form
| If you mean / need | Use | Why |
|---|---|---|
| a reserve, support, or extra copy | backup | The one-word form is commonly a noun or adjective. |
| the action of supporting, copying, moving backward, or accumulating | back up | The two-word form is the phrasal verb. |
Further guidance
Core difference
Use one-word “backup” mainly as a noun or adjective. Use two-word “back up” when it is a verb.
Fast Test
If you can put “to” before it—“to back up the files”—use two words. If you mean the extra copy itself—“a backup”—use one word.
Variant Boundary
Some dictionaries also record hyphenated “back-up” in particular noun/adjective contexts, especially in British/business usage, so house style can affect the non-verb spelling.
Sources and evidence
Sources are shown with the role they play in this guide. Historical or style-sensitive claims are kept within the evidence boundary described above.
- Backup — English meaning (opens in a new tab)Cambridge University Press · Noun and adjective uses of backup
- Back someone/something up — English meaning (opens in a new tab)Cambridge University Press · Phrasal-verb uses of back up