water vapor vs water vapour

water vapor and water vapour both are nouns.

Word	Noun	Adjective	Verb	Adverb
water vapor	Yes	No	No	No
water vapour	Yes	No	No	No

As nouns, water vapor and water vapour are synonyms defined as:

water vapor and water vapour: water in a vaporous form diffused in the atmosphere but below boiling temperature

water vapor (noun)	water vapour (noun)
water in a vaporous form diffused in the atmosphere but below boiling temperature	water in a vaporous form diffused in the atmosphere but below boiling temperature