See P. Roberts, Understanding Grammar (1954) and Modern Grammar (1968); E. Finegan and N. Besnier, Language: Its Structure and Use (1989).
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".
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.
An adjective acts as the head of an adjectival phrase. In the simplest case, an adjectival phrase consists solely of the adjective; more complex adjectival phrases may contain one or more adverbs modifying the adjective ("very strong"), or one or more complements (such as "worth several dollars", "full of toys", or "eager to please"). In English, attributive adjectival phrases that include complements typically follow their subject ("an evildoer devoid of redeeming qualities").
Many languages have special verbal forms called participles that can act as noun modifiers. In some languages, including English, there is a strong tendency for participles to evolve into adjectives. English examples of this include relieved (the past participle of the verb relieve, used as an adjective in sentences such as "I am so relieved to see you"), spoken (as in "the spoken word"), and going (the present participle of the verb go, used as an adjective in sentences such as "Ten dollars per hour is the going rate").
Other constructs that often modify nouns include prepositional phrases (as in English "a rebel without a cause"), relative clauses (as in English "the man who wasn't there"), other adjective clauses (as in English "the bookstore where he worked"), and infinitive phrases (as in English "pizza to die for").
In relation, many nouns take complements such as content clauses (as in English "the idea that I would do that"); these are not commonly considered modifiers, however.
In many languages, adjectives can be compared. In English, for example, we can say that a car is big, that it is bigger than another is, or that it is the biggest car of all. Not all adjectives lend themselves to comparison, however; for example, the English adjective extinct is not considered comparable, in that it does not make sense to describe one species as "more extinct" than another. However, even most non-comparable English adjectives are still sometimes compared; for example, one might say that a language about which nothing is known is "more extinct" than a well-documented language with surviving literature but no speakers.
Comparable adjectives are also known as "gradable" adjectives, because they tend to allow grading adverbs such as very, rather, and so on.
Among languages that allow adjectives to be compared in this way, different approaches are used. Indeed, even within English, two different approaches are used: the suffixes -er and -est, and the words more and most. (In English, the general tendency is for shorter adjectives and adjectives from Anglo-Saxon to use -er and -est, and for longer adjectives and adjectives from French, Latin, Greek, and other languages to use more and most.) By either approach, English adjectives therefore have positive forms (big), comparative forms (bigger), and superlative forms (biggest); many languages do not distinguish comparative from superlative forms, however.
Attributive adjectives, and other noun modifiers, may be used either restrictively (helping to identify the noun's referent, hence "restricting" its reference), or non-restrictively (helping to describe an already-identified noun). In some languages, such as Spanish, restrictiveness is consistently marked; for example, Spanish la tarea difícil means "the difficult task" in the sense of "the task that is difficult" (restrictive), while la difícil tarea means "the difficult task" in the sense of "the task, which is difficult" (non-restrictive). In English, restrictiveness is not marked on adjectives, but is marked on relative clauses (the difference between "the man who recognized me was there" and "the man, who recognized me, was there" being one of restrictiveness).