The spaced form “turn on” functions as a phrasal verb. The noun meaning a person or thing that excites or attracts is commonly written “turn-on”; Collins additionally records a closed American-English noun “turnon,” so the page keeps that variation explicit.
ReviewedEvidence1 sourceSectionEnglish Usage
Quick answer
Use “turn on” as the phrasal verb. For the informal noun, “turn-on” is a standard dictionary form; Collins also records closed “turnon” in American English.
Key details
Confusion Familyword boundary
Rule Familynoun versus phrasal verb
Termsturn-on, turn on, turnon
Variant Scope“Turn on” is the phrasal verb; Collins records hyphenated noun “turn-on” and also a closed American-English noun “turnon.”
Further guidance
Core difference
Use “turn on” as a phrasal verb. Use the noun form “turn-on” when naming something that excites or attracts; Collins also records closed “turnon” in American English.
Fast Test
If you can inflect the action as “turned on” or “turning on,” use the two-word verb. If it functions as a noun, use a dictionary-recorded noun form.
Usage Boundary
The noun is informal/slang in the attraction/excitement sense, and noun spelling varies by dictionary/variety; do not claim the closed noun is universally impossible.
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.
Cambridge distinguishes the two-word phrasal verb “mix up” from the hyphenated noun “mix-up.” The candidate spelling “mixup” is therefore normalized to the dictionary-backed noun form rather than treated as the default closed noun.
“Lockdown” is the noun for an official restriction or state of restricted movement, while “lock down” is the corresponding phrasal verb. Sentence function determines the spacing.
“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.”
“Pinpoint” is a dictionary word used as a verb, noun, and attributive adjective. The two-word sequence “pin point” is appropriate only when “pin” and “point” keep their independent meanings, such as referring literally to the point of a pin.
“Touchdown” and “touch down” differ mainly by grammatical function. Cambridge records the closed form as a countable noun for landing and sports senses and the two-word form as a phrasal verb, including aircraft landing and rugby uses.
“Turn off” is the two-word phrasal verb for switching something off, leaving a road, or causing loss of interest. The corresponding noun appears as closed “turnoff” in American usage and also as hyphenated “turn-off” in Cambridge.
