An object complement is a word or phrase that follows and modifies or renames a direct object in a sentence. It provides additional information about the object, helping to clarify or complete its meaning. For example, in the sentence "They elected John president," the word "president" is the object complement that describes what John has become. Object complements can be nouns, pronouns, or adjectives. They are essential in sentences where the action of the verb transforms or defines the object. In "She painted the wall blue," the word "blue" serves as the object complement, indicating the new state of the wall.