Original: A voluntary association or union (also sometimes called a voluntary organization, unincorporated association, or just an association) is a group of individuals who voluntarily enter into an agreement to form a body (or organization) to accomplish a purpose.
Simplified: A voluntary association or unionalso sometimes called an voluntary organization, unincorporated association, or just an association is a group of individuals who voluntarily enter into an agreement to form a body (or organization) to work together for a purpose.