water vapour vs water vapor

water vapour and water vapor both are nouns.

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

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

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

water vapour (noun)	water vapor (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