These are terms that I’ve used somewhat equivalently in everyday conversations to designate a non-democratic government, however I thought it useful to elucidate the differences.
Authoritarianism
This is just the umbrella term for any system which is centralized around a centralized authority. If all else fails, this is the general term for a form of government with concentrated power.
Autocracy
Autocracy is a system of government where power is particularly concentrated in the head of state, or the autocrat. Related to authoritarianism, it’s focused around one person. It could be a legal or hereditary form.
Dictatorship
This is a form of rule where power is focused into a single person or a group of people. Typically, power is seized through a coup. The dictator has to maintain power by appeasing an inner circle of elites which facilitate his rule, e.g., military leaders and oligarchs. Force is often used to maintain control.
Totalitarianism
By far, this is the most extreme form of authoritarian government. Totalitarian rule seeks to control everything, up to and including its subjects’ thoughts. Both the public and the private spheres are controlled by the government. The dictator is typically seen as charismatic, with a cult of personality around them. Mass surveillance, political repression, and every other tool is used to control. The USSR under Stalin is considered as such. North Korea is another.
Absolute Monarchy
An absolute monarchy is a form of autocracy in which power is passed down through a royal family line. The monarch controls all branches of government and is above any law or constitution.
Tyranny
Tyranny is more about how power is used. It designates states that exercise power in a cruel and oppressive manner.