In grammar, a complement is a word, phrase, or clause that is necessary to complete the meaning of a given expression. Complements are often also arguments (expressions that help complete the meaning of a predicate).

In many non-theoretical grammars, the terms subject complement (also called a predicative of the subject) and object complement are employed to denote the predicative expressions (predicative complements), such as predicative adjectives and nominals (also called a predicative nominative or predicate nominative), that serve to assign a property to a subject or an object:

This terminology is used in grammar books:

However, this use of terminology is avoided by many modern theories of syntax, which typically view the expressions in bold as part of the clause predicate, which means they are not complements of the subject or object but rather are properties that are predicated of the subject or object.

The Cambridge Grammar of the English Language assigns the term "predicative complement" to both uses and shifts the terminological distinction to the verb:

In many modern grammars (for instance in those that build on the X-bar framework), the object argument of a verbal predicate is called a complement. In fact, this use of the term is the one that currently dominates in linguistics. A main aspect of this understanding of complements is that the subject is usually not a complement of the predicate:

While it is less common to do so, one sometimes extends this reasoning to subject arguments:

In those examples, the subject and object arguments are taken to be complements. In this area, the terms complement and argument thus overlap in meaning and use. Note that this practice takes a subject complement to be something very different from the subject complements of traditional grammar, which are predicative expressions, as just mentioned above.