union

The word union refers to the act or result of joining two or more things into a single entity. It is used in everyday language as well as in technical contexts.

In set theory and mathematics, the union of a collection of sets is the set of elements that belong to at least one member. The union operation is denoted by the symbol ∪ and is both commutative and associative. Finite unions combine a finite number of sets; infinite unions may require careful handling in terms of definitions and cardinality. For two sets A and B, A ∪ B contains all elements that are in A or in B or in both.

In labor history, a union is an organization that represents workers in negotiations with employers over wages, working conditions, and benefits. Unions engage in collective bargaining and may organize strikes or political advocacy. They vary by country in scope, power, and legal framework.

In programming, a union is a data structure that can store different types of data in the same memory location, with a requirement to keep track of the active type. In C, a union holds one of its member types at a time, while other languages support tagged or discriminated unions that carry type information.

In politics and governance, unions can refer to federations or confederations of states or territories, such as a union government or union of countries, often emphasizing shared institutions while preserving some sovereignty. The term can also be used more loosely to describe a coalition focused on common goals.