Generally speaking, let S be a set, and A be a subset of S, which is composed of all elements in S that do not belong to A. It is called the complement of subset A in S (or CsA). It is read as the complement of a in s.

In mathematics, especially in the application of set theory and mathematical foundation, the whole class (if it is a set, it is a complete set) is about such a class, which (to some extent) contains all the research objects and sets.

Mathematically, generally speaking, the intersection of two given sets A and B refers to the set that contains all the elements belonging to A and B, but does not contain other elements.

Generally speaking, for a given two sets A and B, the set consisting of all elements belonging to set A or set B (the set consisting of all elements added by all elements of two sets) is called union, marked as A∪B, and pronounced as "A and B".

A∪B={xIx∈A or x∈B}