vapour pressure vs vapor pressure

vapour pressure and vapor pressure both are nouns.

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

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

vapour pressure and vapor pressure: the pressure exerted by a vapor; often understood to mean saturated vapor pressure (the vapor pressure of a vapor in contact with its liquid form)

vapour pressure (noun)	vapor pressure (noun)
the pressure exerted by a vapor; often understood to mean saturated vapor pressure (the vapor pressure of a vapor in contact with its liquid form)	the pressure exerted by a vapor; often understood to mean saturated vapor pressure (the vapor pressure of a vapor in contact with its liquid form)