seatbelt vs seat belt

seatbelt and seat belt both are nouns.

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

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

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

seatbelt (noun)	seat belt (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