Dual is a grammatical number referring to exactly two referents. Duals only exist in languages that also have a singular and plural number. A number between these that can sometimes refer to more than two referents is called a paucal.
As with other number distinctions, a language can have a systematic dual category extending to all pronouns, nouns, possession, verbal conjugation etc, or only a subset of these. The subset in question can be smaller than the subset in which a distinction between singular and plural is made, for example distinguishing dual only in pronouns.