A tyrant (pronounce: tie-rant) is a person who rules with absolute power. In its Greek origin the word has no negative meaning: we translate Oedipus Tyrannus as 'Oedipus the King'. A tyrant usually rules a country, and he often got his position as powerful ruler by force, although some of them inherited their power.
Later, the word came to mean someone who ruled with cruelty and injustice. The rule of a tyrant is called tyranny. The adjective is tyrannical.