A potentate is a person so powerful they don't have to follow the rules that govern everyone else. Potentate normally refers to a king or dictator, but you can call anyone with virtually unlimited power a potentate.
The king of a country, the conductor of an orchestra, the commander of a battleship—all of these are examples of a potentate. Take a look at potentate, and you'll see the word potent, which means "powerful," as in "that's one potent cup o' joe!" It's easy to see, then, how potent becomes potentate just by adding a few letters. A potentate is a powerful person. Anna Wintour is a potentate of the fashion world, and her decisions can make or break whole careers.