A predicate adjective is an adjective that follows a connecting or linking verb that describes the subject of the sentence or phrase. Technically, as a rule of grammar, this is known as a an adjective used in the predicate sense.
A simple example of a predicate adjective is found in the phrase "violets are blue." "Violets" represents the subject, "are" the linking verb and "blue" is the predicate adjective. Another example of a predicate adjective is found in the phrase "she is happy," with "happy" being the predicate adjective. The identification of a predicate adjective as a part of speech is thought to have developed sometime between 1880 to 1885.
Some adjectives can only be predicative. These include adjectives that begin with the a- prefix such as "asleep," health adjectives such as "well" or "faint" and adjectives that must be followed by a prepositional phrase or infinitive such as "inclined to disagree." Predicative adjectives also appear in front of a subject noun when used as appositives, as in the sentence, "Slender and graceful, the ballerina seemed to float across the stage."