1. A male sovereign or head of state, esp. one ruling an empire.
2. (in the Holy Roman Empire) the title borne by the elected head of state, who was often also king of Germany and Holy Roman Emperor.
3. (in ancient Rome) the title of the supreme ruler, originally bestowed on victorious generals and later adopted by Julius Caesar and his successors.
4. (in China and Japan) the title of the head of state, used until the 20th century.