“adjective: Defined traditionally as a word added to a noun, which characteristically denotes a property of whoever or whatever is referred to. One function therefore is as a modifier: e.g. tall in tall men is an adjective modifying men. Another is in predicative position: e.g. that of tall in These men are tall.
Adjectives were included in antiquity in the same part of speech as nouns. Distinguished in the later Middle Ages, as ‘nouns adjective’ in contrast to ‘nouns substantive’; and so called, still, in the early decades of the 20th century.
An adjectival element is on either forming or having the role of adjectives: e.g. -less in clueless is an adjectival affix; English participial adjectives in -ed, such as interested in very interested, have been called ‘adjectival passives.”
Excerpted from: Matthews, P.H., ed. The Oxford Concise Dictionary of Linguistics. New York: Oxford University Press, 2014.