In descriptions of the Japanese language, an adjectival noun, adjectival, or na-adjective is a noun that can function as an adjective by taking the particle 〜な -na. (In comparison, regular nouns can function adjectivally by taking the particle 〜の -no, which is analyzed as the genitive case.) Adjectival nouns constitute one of several Japanese word classes that can be considered equivalent to adjectives.
In their attributive function, Japanese adjectival nouns function similarly to English noun adjuncts, as in "chicken soup" or "winter coat" – in these cases, the nouns "chicken" and "winter" modify the nouns "soup" and "coat", respectively. Japanese adjectival nouns can also be used predicatively – in that use, they do not take the -na suffix, but normally combine with forms of the copular verb.
Terminology
The terminology used to refer to this word-class is inconsistent. The Japanese name is keiyō dōshi, which literally means "adjectival verb". This is not necessarily at odds with the English term adjectival noun, since in traditional Japanese grammar, keiyō dōshi includes the copula, while the adjectival noun in the analysis described here does not include the copula. For example, in the traditional grammar, kirei da is a keiyō dōshi and kirei is its stem; in the analysis here, kirei is an adjectival noun and kirei da is its combination with the copula. Considering the copula is a kind of verb and kirei is a kind of noun syntactically, both terminologies make sense.
Grammatically, these words are similar to nouns, or more technically, nominals, which function attributively (like adjectives). The main differences between these and regular nouns are that nouns take a 〜の -no suffix when acting attributively, while these words take a 〜な -na suffix when acting attributively; most of these words cannot be used as the agent or patient (i.e. subject) of a sentence; and most of these words cannot be modified by relative clauses. Aside from these differences, Japanese adjectival nouns and regular nouns behave the same way in terms of grammar. They are variously referred to as "adjectival verbs" (literal translation), "adjectival nouns" (nouns that function adjectivally), "na adjectives" (function as adjectives, take na), and "na nominals" (nominals that take na). For example, Eleanor Harz Jorden refers to them as "na-nominals" in her textbook Japanese: The Spoken Language.
In fact, by some analyses, nouns and na-nominals are fundamentally grammatically the same, where 〜の vs. 〜な when used attributively is simply a conventional stylistic complementary distribution, with 〜の／〜な being allomorphs. This view is reinforced by the fact that some words, such as 特別 tokubetsu "special", can take either a 〜の or a 〜な, depending on the phrase. Ultimately, 〜な is an abbreviation of 〜にある, used to use a noun attributively (compare modern 〜である, which is a more recent form), while 〜の is the genitive case; see etymology, below.
Characterization
In traditional Japanese grammar, adjectival nouns are considered "inflectional", katsuyō, like verbs and i-adjectives, rather than non-inflectional hikatsuyōgo (非活用語) or mukatsuyōgo (無活用語), like nouns.
This is a point of disagreement in current Japanese grammar, and authors such as Uehara (1998) argue that instead, adjectival nouns should be classed with nouns as non-inflectional.
The claim that na-adjectives are inflectional rests on the claim that the syllable da 'is', usually regarded as a "copula verb", is really a suffix – an inflection. Thus, hon 'book', generates a one-word sentence, honda 'it is a book', not a two-word sentence, hon da. However, numerous constructions seem to be incompatible with the suffixal copula claim.
(1) Reduplication for emphasis
Hora! Hon, hon! 'See, it is a book!'
Hora! Kirei, kirei! 'See, it is pretty!'
Hora! Furui, furui! 'See, it is old!' (the adjectival inflection -i cannot be left off) Hora! Iku, iku! 'See, it does go!' (the verbal inflection -u cannot be left off)
(2) Questions. In Japanese, questions are formed by adding the particle ka (or in colloquial speech, just by changing the intonation of the sentence). Hon/kirei ka? 'Is it a book? ; Is it pretty?' Furu-i/Ik-u ka? 'Is it old? ; Does it go?' (the inflections cannot be left off)