模糊集是模糊数学上的一个基本概念，是数学上普通集合的扩展。

给定一个论域 U ，那幺从 U 到单位区间 [0,1] 的一个映射 称为 U 上的一个模糊集，或 U 的一个模糊子集，

In mathematics, fuzzy sets are sets whose elements have degrees of membership. Fuzzy sets were introduced by Lotfi A. Zadeh and Dieter Klaua in 1965 as an extension of the classical notion of set. At the same time, Salii (1965) defined a more general kind of structure called an L-relation, which he studied in an abstract algebraic context. Fuzzy relations, which are used now in different areas, such as linguistics (De Cock, Bodenhofer & Kerre 2000) decision-making (Kuzmin 1982) and clustering (Bezdek 1978), are special cases of L-relations when L is the unit interval [0, 1].