complementizer

In linguistics (especially generative grammar), complementizer or complementiser is a lexical category (part of speech) that includes those words that can be used to turn a clause into the subject or object of a sentence. For example, the word that may be called a complementizer in English sentences like Mary believes that it is raining. The concept of complementizers is specific to certain modern grammatical theories; in traditional grammar, such words are normally considered either conjunctions or relative pronouns.