In grammar, an adjective is a word whose main syntactic role is to modify a noun or pronoun, giving more information about the noun or pronoun's referent. Some examples can be seen in the box to the right. Collectively, adjectives form one of the traditional eight parts of speech, though linguists today distinguish adjectives from words such as determiners that also used to be considered adjectives.

Not all languages have adjectives, but most, including English, do. (English adjectives include big, old, and tired, among many others.) Those that do not typically use words of another part of speech, often verbs, to serve the same semantic function; for example, such a language might have a verb that means "to be big", and would use a construction analogous to "big-being house" to express what English expresses as "big house". Even in languages that do have adjectives, one language's adjective might not be another's; for example, while English uses "to be hungry" (hungry being an adjective), French uses "avoir faim" (literally "to have hunger"), and where Hebrew uses the adjective "זקוק" (zaqūq, roughly "in need of"), English uses the verb "to need".

In most languages with adjectives, they form an open class of words; that is, it is relatively common for new adjectives to be formed via such processes as derivation.

Adjectives and adverbs

Many languages, including English, distinguish between adjectives, which describe nouns and pronouns, and adverbs, which modify verbs, adjectives, and other adverbs. Not all languages have exactly this distinction, however, and in many languages (including English) there are words that can function as both. For example, English fast is an adjective in "a fast car" (where it modifies the noun car), but an adverb in "he drove fast" (where it modifies the verb drove).

Determiners

main: Determiner (class)

Linguists today distinguish determiners from adjectives, considering them to be two separate parts of speech (or lexical categories), but traditionally, determiners were considered adjectives in some of their uses. (In English dictionaries, which typically still do not treat determiners as their own part of speech, determiners are often recognizable by being listed both as adjectives and as pronouns.) Determiners are words that express the reference of a noun in the context, generally indicating definiteness (as in a vs. the), quantity (as in one vs. some vs. many), or another such property.

Attributive, predicative, absolute, and substantive adjectives