The word Unionist, simply meaning one espousing a union, has a number of connotations, depending on context:
Unionists are people in Ireland, England, Scotland and Wales who were historically in favour of uniting their nations into a United Kingdom, or who in modern times wish their nations to remain part of the United Kingdom.
Members and supporters of trades unions also bear the label of unionists.
Unionist is also a term used to describe someone loyal to the Union Forces during the American Civil War. For greater detail on American Unionists see the article Unionists (American).
