According to the Cambridge and Merriam-Webster dictionaries, a person who takes power by force and rules with total authority is a dictator. A dictator is not elected by the people, and extends his powers into the private lives of the people. He usually enjoys a cult of personality throughout the nation. An example of dictator would be Hitler in Germany between 1933 and 1945.
You should bear in mind that there are two other words which could fit this definiton, with some nuance:
- if this ruler uses its unlimited power unfairly and/or cruelly, it is a tyrant;
- if this ruler came to power through elections which he rigged, and simply rules without any political opposition, it is an autocrat.
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The Nazis used the theory of evolution as justification for their persecution of the Jewish people. In their mind, Jewish people were "corrupting" the non-Jewish bloodstream. This is what led to the reprehensible eugenics movement.
