Authoritarianism, dictatorship, and despotism are more or less synonyms, referring to a government or political system that centralises power, represses opposition, restricts political freedoms, and is ruled by and for an individual or small group of people. Dictator and despot are single individuals, so dictatorship and despotism are more about a single person calling the shots. Authoritarianism could have a single person at the top, but an authoritarian regime could also have a group of people, like a clique of military officers, running things.