potentate \POH-tuhn-tayt\ (noun) - One who possesses great power or sway; a ruler, sovereign, or monarch.
"When the Vizerine has been much younger, she had been taken a slave for three weeks and forced to perform services arduous and demeaning for a provincial potentate -- who bore such a resemblance to her present cook at Court that it all but kept her out of the kitchen." -- Samuel R. Delany, 'Tales of Neveryon'
Potentate derives from Late Latin potentatus, "a powerful person," from Latin potentatus, "power, especially political power; supremacy," from potens, "able, powerful," from posse, "to be able." It is related to potent, "powerful," and potential, "having possibility or capability."