1a Protestant in Northern Ireland who believes that it should remain part of the UK. The people who want Northern Ireland to become part of the Republic of Ireland are called 'Republicans' or 'Nationalists'.
2someone who was loyal to the royal family during the English Civil War
3someone who supported the elected government during the Spanish Civil War
4someone who fought with the British during the American Revolutionary War
5someone who supported the Union during the American Civil War
• the Loyalist flag
Definition from the Longman Dictionary of Contemporary English Advanced Learner's Dictionary.