Explanation

When an adjective describes a noun that is part of the predicate, it is called an object complement.

An object complement is a word or phrase that renames or describes the direct object of a verb.

It provides more information about the object, such as its qualities, properties, or characteristics.

In the sentence "They elected him president," "president" is the direct object, and the adjective "president" is the object complement, renaming or describing the direct object "him".