monarchy

Short definition from Wiktionary

Noun

1. 1
   A government in which sovereignty is embodied within a single, today usually hereditary head of state (whether as a figurehead or as a powerful ruler).
2. 2
   The territory ruled over by a monarch; a kingdom.
3. 3
   A form of government where sovereignty is embodied by a single ruler in a state and his high aristocracy representing their separate divided lands within the state and their low aristocracy representing their separate divided fiefs.
4. 4
   States based on a system of governance headed by a king or a queen.