seat belt vs seatbelt

seat belt and seatbelt both are nouns.

Word	Noun	Adjective	Verb	Adverb
seat belt	Yes	No	No	No
seatbelt	Yes	No	No	No

As nouns, seat belt and seatbelt are synonyms defined as:

seat belt and seatbelt: a safety belt used in a car or plane to hold you in your seat in case of an accident

seat belt (noun)	seatbelt (noun)
a safety belt used in a car or plane to hold you in your seat in case of an accident	a safety belt used in a car or plane to hold you in your seat in case of an accident