Unionists are a group of people in Ireland and Scotland who wish their respective countries (and quite often both countries) to remain part of the United Kingdom.
For greater detail on Unionists in Ireland see the article Unionists (Ireland), and for greater detail on Unionists in Scotland see the article Unionists (Scotland).
Members and supporters of trade unionss 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).
