**Fuzzy sets** are an extension of the classical set theory. A fuzzy set is characterized by a *membership-degree function*, which maps the members of the Universe into the unit interval [0,1]. The value 0 means that the member is not included in the given set, 1 describes a fully included member (this behaviour corresponds to the classical sets). The values between 0 and 1 characterize fuzzy members.

For the universe *X* and given the membership-degree function *f* : *X*→[0;1], the fuzzy set *A* is defined as *A* = {(*x*, *f*(*x*)) | x ∈ *X*}.

Related to Fuzzy logic.