Adjectival means "working like an adjective" or "about adjectives". An adjective is a word that tells you more about a noun, or about a word like she or it: in "black hat", black describes the hat; in "It makes her happy", happy describes her. Though actually, it is not only single words that can do this job. A whole little group of words can sit in the adjective's place, and then we call that group adjectival — "the girl with red shoes", "a never-to-be-forgotten day". So adjectival is a grammar teacher's word. You use it when you want to talk about the job (describing a person or thing), not only the word class "adjective". It is formal. Ordinary conversation almost never needs it; adjective is the word students meet first, and for most days it is enough.

形容詞的；作形容詞用的（文法用語：指用來描述名詞的詞或片語）