What is the meaning of Adjectival_noun?

A noun used as an adjective.

A specific part of speech, categorized in Japanese as 形容動詞 (keiyō dōshi). Some of these words can be used as regular nouns, and all can be used as adjectives when followed by the postfix な (na), in contrast to Japanese common adjectives, 形容詞 (keiyōshi).

Nouns that have the form of adjectives (often originally adjectives used as nouns).

A nominalized adjective.

Source: wiktionary.org