An adjective is a member of a word class whose members most typically express properties.
The corresponding relational adjective is adjectival.
Typical adjective meanings are 'old', 'young', 'big', 'small', 'good', 'bad', 'red', 'blue', 'quick'.
The term adjective goes back to antiquity, Latin (nomen) adjectivum, literally 'attached (noun)'. (In the earlier grammatical terminology, adjectives were a subclass of noun; see noun (Latin nomen).)
Dixon, R. M. W. & Aikhenvald, Alexandra Y. (eds.) 2004. Adjective classes: A cross-linguistic typology. Oxford: Oxford University Press.