fuzzy set theory
an approach to set theory that allows gradations of membership in a set, instead of only assigning a 0 or a 1 that would indicate nonmembership or absolute membership, respectively. Some phenomena are more readily modeled with fuzzy set theory than conventional binary set theory. For example, in designation of a mental illness, fuzzy set theory may be employed to allow for cases in which individuals could be classified somewhere between having an absence and a full diagnosis of a particular condition.