A credit union is a cooperative financial institution that is owned and controlled by its members, and operated for the purpose of promoting thrift, providing credit at reasonable rates, and providing other financial services to its members. Many credit unions exist to further community development or sustainable international development on a local level. Worldwide, credit union systems vary significantly in terms of total system assets and average institution asset size since credit unions exist in a wide range of sizes, ranging from volunteer operations with a handful of members to institutions with several billion dollars in assets and hundreds of thousands of members. Yet credit unions are typically smaller than banks; for example, the average U.S. credit union has $93 million in assets, while the average U.S. bank has $1.53 billion, as of 2007.

The World Council of Credit Unions (WOCCU) defines credit unions as "not-for-profit cooperative institutions." In practice however, legal arrangements vary by jurisdiction. For example in Canada credit unions are regulated as for-profit institutions, and view their mandate as earning a reasonable profit to enhance services to members and ensure stable growth. This difference in viewpoints reflects credit unions' unusual organizational structure, which attempts to solve the principal-agent problem by ensuring that the owners and the users of the institution are the same people. In any case, credit unions generally cannot accept donations and must be able to prosper in a competitive market economy.