complement: an element of a phrase, clause or other complex structure which completes the meaning of the head, in effect answering a crucial question that the head alone would leave unanswered. The subject and object of a verb, and the object of an adposition, are typical complements. For instance, in the clause "John saw Charlie yesterday at the store", "John" and "Charlie" are complements of "saw", because they answer the burning questions raised by "saw" (who saw whom?), whereas "yesterday" and "at the store" are modifiers of "saw", answering more contingent or peripheral questions (exactly when? where?).

Verbs can also have adverbial or adpositional complements. For example, in the sentence "John put the book on the table", the locative prepositional phrase "on the table" functions as a complement, because the meaning of "put" is such that the question "where?" is central to it, in contrast to the same question in the case mentioned above with "saw". It is for this reason that the locative phrase can be omitted in the first example and still leave a perfectly well-formed clause: "John saw Charlie (yesterday)". In contrast, if the locative phrase is omitted from the second example, the result does not sound normal: "?John put the book".

Contrast modifier; compare argument. [Spanish: complemento]