A voluntary association or union (also sometimes called a voluntary organization, unincorporated association, common-interest association, or just an association) is a group of individuals who enter into an agreement as volunteers to form a body (or organization) to accomplish a purpose. Common examples include trade associations, trade unions, learned societies and professional associations, environmental groups, and various other types of groups. Membership is not necessarily voluntary, as it may be effectively required in order to work, which has led to a preference for the term common-interest association to describe groups which form out of a common interest. Associations may also be incorporated rather than unincorporated; for example, in the United States associations gained additional powers by incorporating.
Strictly speaking, in many jurisdictions no formalities are necessary to start an association. In some jurisdictions, there is a minimum for the number of persons starting an association. Some jurisdictions require that the association register with the police or other official body to inform the public of the association's existence. This could be a tool of political control, and also a way of protecting the economy from fraud. In many such jurisdictions, only a registered association is a juristic person whose membership is not responsible for the financial acts of the association. Any group of persons may, of course, work as an association but in such case, the persons making a transaction in the name of the association all take responsibility for it.
Voluntary associations are a broad and original form of nonprofit organizations, and have existed since ancient history. In Ancient Greece, for example, there were various organizations ranging from elite clubs of wealthy men (hetaireiai) to private religious or professional associations.