Phase Out vs. Phaseout
“Phase out” and “phaseout” differ by grammatical function. Merriam-Webster records “phaseout” as the noun and “phase out” as the corresponding verb, with forms such as “phased out” and “phasing out.”
Use “phase out” as the verb meaning to discontinue gradually; use “phaseout” as the noun for the gradual discontinuation.
Key details
Choose the right form
| Meaning / situation | Form / construction | Decision |
|---|---|---|
| action of discontinuing something gradually | phase out | Use two words for the verb. |
| the gradual discontinuation as a thing or process | phaseout | Use the closed noun. |
| inflected verb | phased out / phasing out | The spacing stays with the verb. |
Quick tests
Meaning / construction test
Can the expression take verb tense or inflection in the sentence?
If yes: Use the two-word verb “phase out.”
If no: Use the noun “phaseout” when you are naming the process or event itself.
Examples
The company will phase out the older model.
The action is expressed by the two-word verb.
The phaseout will take several months.
The closed form is the noun naming the process.
Several features are being phased out.
The inflected verb remains two words.
Further guidance
Core difference
Use one-word “phaseout” for the process/event as a noun; use two-word “phase out” for the action as a verb.
Fast Test
If the phrase can inflect as “phased out” or “phasing out,” you need the two-word verb.
Usage Boundary
Hyphenation and house style can vary in modifiers, but the core noun-versus-verb distinction should remain clear.
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.
- phaseout — Merriam-Webster.com Dictionary (opens in a new tab)Merriam-Webster · Direct dictionary evidence distinguishing noun phaseout from verb phase out and documenting their inflected verb forms.