The Loyalists were Americans who fought on the side of the British during the American Revolution.
According to the United Empire Loyalists' Association of Canada:
"Perhaps as many as 50 000 or more individuals were Loyalists - estimates range from 30,000 to over 100,000. To be a Loyalist, one had to have been:
Either male or female, as of 19 April 1775, a resident of the American colonies, and joined the Royal Standard prior to the Treaty of Separation of 1783, or otherwise demonstrated loyalty to the Crown, and settled in territory remaining under the rule of the Crown;
or a soldier who served in an American Loyalist Regiment and was disbanded in Canada;
or a member of the Six Nations of either the Grand River or the Bay of Quinte Reserve who is descended from one whose migration was similar to that of other Loyalists.
However, there were others who qualified too -- for example, if a Loyalist was killed in action, and his family then moved to Canada and settled around the time of the Treaty of Separation in 1783, they might qualify."