Adjectives in English grammar are traditionally defined as “words that describe nouns.”
Adjective is a grammatical form.
Adjectives function as the heads of adjective phrases. The three grammatical functions performed by adjective phrases, and subsequently adjectives, are noun phrase modifier, subject complement, and object complement.
The internal structures, or grammatical form, that distinguish adjectives from other grammatical forms include degrees of modification.
For information about forming comparative and superlative adjectives, see Positive, Comparative, and Superlative Adjectives.
Brinton, Laurel J. & Donna M. Brinton. 2010. The linguistic structure of Modern English, 2nd edn. Amsterdam: John Benjamins Publishing Company.
Hopper, Paul J. 1999. A short course in grammar. New York: W. W. Norton & Company.
Huddleston, Rodney. 1984. Introduction to the grammar of English. Cambridge: Cambridge University Press.