One who reigns over a state or territory, usually for life and by hereditary right, especially: A sole and absolute ruler. A sovereign, such as a king or empress, often with constitutionally limited authority: a constitutional monarch. One that commands or rules: "I am monarch of all I survey\u201d ( William Cowper). One that surpasses others in power or preeminence: "Mont Blanc is the monarch of the mountains\u201d ( Byron). A monarch butterfly.