Relativizer
In linguistics, a relativizer (abbreviated REL) is a type of conjunction that introduces a relative clause. For example, in English, the conjunction that may be considered a relativizer in a sentence such as "I have one that you can use." Relativizers do not appear, at least overtly, in all languages; even in languages that do have overt or pronounced relativizers, they do not necessarily appear all of the time. For these reasons it has been suggested that in some cases, a "zero relativizer" may be present, meaning that a relativizer is implied in the grammar but is not actually realized in speech or writing. For example, the word that can be omitted in the above English example, producing "I have one you can use", using (on this analysis) a zero relativizer. (Wikipedia).
