Uniqueness is a property ascribed to a particular object in a set concerning a property possessed by said object, but not by any other. For example, zero is the *unique* additive identity in the real numbers because it is precisely the only real number that serves as the additive identity.

Uniqueness is often used in conjunction with the existential quantifier in predicate logic, and symbolized by . Given a set with a property , we express the existence and uniqueness of an element satisfying by:

- ("There exists a unique in such that ")

To reduce the language to simple existential and universal quantification, we recognize that there is indeed an element satisfying (itself an existential statement) and that for any element , if also satisfies , then must be the same as :

- .

An equivalent definition that is more commonly used in proof separates the notions of existence and uniqueness together, first by expressing the existence of satisfying and then expressing that any two elements satisfying must actually be the same:

- .