A mutineer is someone who joins a mutiny, which is an organized rebellion against authority, especially aboard a ship or in the military. When sailors mutiny against their captain, they're refusing to obey orders and often trying to take control of the vessel themselves. Each person participating in that rebellion is a mutineer.

The most famous mutineers in history sailed on HMS Bounty in 1789. Led by Fletcher Christian, the mutineers seized the ship from Captain William Bligh in the South Pacific. Their story became legendary, spawning books and movies, though the reality was far more complex than the adventure tales suggest.

Mutiny was considered one of the most serious crimes in naval history because ships at sea depended completely on order and discipline for survival. A crew of mutineers could leave their ship helpless in a storm or stranded thousands of miles from land.

While we mostly hear about mutineers in historical naval contexts, the word can describe anyone who rebels against established authority in an organized way, though it's rarely used outside maritime or military settings.