adjective, adjectival: a word that modifies a noun or nominal phrase. An adjectival phrase is a phrase that similarly functions as a noun modifier. For example, in the phrase "a boring book", the word "boring" is an adjective, and modifies "book". In "a very boring book", "very boring" is an adjectival phrase. Adjectival clauses are usually called relative clauses. [Spanish: adjetivo, adjetival]