Definition of COMMON ADJECTIVE. : a descriptive adjective that is not a proper adjective.ADVERTISEMENT. This word doesn't usually appear in our free dictionary, but the definition from our premium Unabridged Dictionary is offered here on a limited basis. Note that some information is displayed differently in the Unabridged. a descriptive adjective that is not a proper adjective. ADVERTISEMENT. This word doesn't usually appear in our free dictionary, but the definition from our premium Unabridged Dictionary is offered here on a limited basis. Note that some information is displayed differently in the Unabridged.