Fenian is a general term used since the 1860s for an Irish nationalist who espouses violence. It can also specifically refer to the Fenian Brotherhood.

It is also used in Northern Ireland as a general derogatory term for Catholics. This usage can also be found in Glasgow, particularly during Old Firm football matches.

In 1865-66, following the US Civil War, Irish-Americans who were former soldiers of the Union Army launched raids into British North American territory in the Province of Canada in order to put pressure on the United Kingdom to leave Ireland. These were known as the Fenian Raids.

Fenian is also the name of the horse that won the Belmont Stakes in 1869. His image appears atop the trophy awarded to the winner of the Belmont Stakes.