“Lookout” is a noun for a person who watches for danger or a place used for watching. “Look out” is a two-word phrasal verb meaning to watch carefully or be careful, and it is also used as a warning.
ReviewedEvidence2 sourcesSectionEnglish Usage
Quick answer
Use “lookout” for the noun and “look out” for the phrasal verb or warning.
Key details
Confusion Familyword-boundary
Noun Variant ScopeCambridge separates the closed noun “lookout” from the two-word phrasal verb “look out.”
Rule Familyorthographic-boundary
Termslookout, look out
Choose the right form
Decision table for Lookout vs. Look Out
If you mean / need
Use
Why
a watcher, observation position, or lookout noun
lookout
The noun is written as one word.
the action or warning expression
look out
The phrasal verb is written as two words.
Further guidance
Core difference
Use “look out” when the expression functions as a verb; use the documented noun form for the person, thing, event, result, or modifier.
Fast Test
If tense or inflection works naturally—such as a past or -ing form—you are using the verb and should keep “look out” as separate words.
Variant Boundary
Cambridge separates the closed noun “lookout” from the two-word phrasal verb “look out.”
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.
Standard English writes “a lot” as two words for a large amount, number, frequency, or degree. “Alot” is a common misspelling in this sense; it should not be confused with the separate verb “allot.”
“Blackout” is the closed noun, while “black out” is the two-word phrasal verb. The verb can describe losing consciousness or obscuring/darkening something; the noun names the resulting event or state.
“Blowout” is a closed noun for several events or results, and Cambridge also records “blow-out” as a noun variant. “Blow out” is the two-word phrasal verb, including the sense of extinguishing a flame.
“Breakout” is a closed noun for an escape and an adjective in several established senses. Cambridge also documents “break-out” in modifier use. “Break out” is the two-word phrasal verb for meanings such as beginning suddenly or escaping.
“Breakthrough” is a closed noun for an important discovery or advance and can also modify a following noun. “Break through” is the two-word phrasal verb for forcing or moving through a barrier or threshold.
“Build up” is the two-word phrasal verb. For the noun, Cambridge records hyphenated “build-up,” while its American dictionary also records closed “buildup.” The noun spelling therefore has a documented regional/style variant rather than one universal form.
