tyrant

Short definition from Wiktionary

Noun

1. 1
   A usurper; one who gains power and rules extralegally, distinguished from kings elevated by election or succession.
2. 2
   Any monarch or governor.
3. 3
   A despot; a ruler who governs unjustly, cruelly, or harshly.
4. 4
   Any person who abuses the power of position or office to treat others unjustly, cruelly, or harshly.
5. 5
   A villain; a person or thing who uses strength or violence to treat others unjustly, cruelly, or harshly.
6. 6
   The tyrant birds, members of the family Tyrannidae, which often fight or drive off other birds which approach their nests.

Adjective

1. 1
   Tyrannical, tyrannous; like, characteristic of, or in the manner of a tyrant.

Verb

1. 1
   To act like a tyrant; to be tyrannical.
2. 2
   To tyrannize.