Fallout vs. Fall Out
“Fallout” is a closed noun with meanings including radioactive dust and adverse consequences. “Fall out” is a two-word phrasal verb with different verb senses, including becoming detached and quarrelling.
Use “fallout” as a noun for radioactive dust or the unpleasant consequences of an event. Use “fall out” as a phrasal verb for becoming detached or for having an argument and no longer being friendly.
Key details
Choose the right form
| If you mean / need | Use | Why |
|---|---|---|
| unpleasant consequences or radioactive dust | fallout | Cambridge records the closed noun. |
| something becoming detached, or people quarrelling | fall out | Cambridge records the phrasal verb as two words. |
Further guidance
Core difference
Use the documented noun or modifier form when naming a thing, result, event, or concept; use the spaced form when the words function as a verb.
Fast Test
If the expression can take ordinary verb tense or inflection in the sentence, the spaced verb form is usually the relevant choice.
Variant Boundary
Cambridge records closed “fallout” as a noun and “fall out” as a phrasal verb; the meanings are related only in spelling, not by one universal noun/verb conversion.
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.
- FALLOUT — English meaning (opens in a new tab)Cambridge Dictionary · Closed noun “fallout”
- FALL OUT — English meaning (opens in a new tab)Cambridge Dictionary · Two-word phrasal verb “fall out”