A dictator was the absolute ruler of a government.
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)
The Twelfth Doctor once compared dictators to tantrumming children. (TV: The Zygon Inversion)