Voluntary association

A voluntary group or union (also sometimes called a voluntary organization, common-interest association, association, or society) is a group of individuals who enter into an agreement, usually as volunteers, to form a body (or organization) to accomplish a purpose. Common examples include trade associations, trade unions, learned societies, professional associations, service organizations, and environmental groups. All such associations reflect freedom of association in ultimate terms (members may choose whether to join or leave), although membership is not necessarily voluntary in the sense that one's employment may effectively require it via occupational closure. For example, in order for particular associations to function effectively, they might need to be mandatory or at least strongly encouraged, as is true of trade unions.