vapor pressure vs vapour pressure

vapor pressure and vapour pressure both are nouns.

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

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

vapor pressure and vapour 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)

vapor pressure (noun)	vapour 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)