tyrant

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

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

adjective

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