A dictator was the absolute ruler of a government.
Joseph Stalin was a communist dictator of the Soviet Union. He served as the General Secretary of the Communist Party of the Soviet Union. (PROSE: History 101)
Jeremiah Maru-Stahl fomented a rebellion on his planet and overthrew the ruling Imperial Family. He became a dictator, then was overthrown himself by the Eighth Doctor. (PROSE: Gazing Void)
On a parallel Earth visited by the Third Doctor, Britain was ruled by a dictator. (TV: Inferno) The Seventh Doctor later discovered that the dictator was an alternative regeneration that the Second Doctor had chosen after his trial by the Time Lords just prior to his exile to Earth. (PROSE: Timewyrm: Revelation)