One who has the power and position to rule over others; a monarch.
One who dominates or leads a group or an endeavor: industrial potentates.
[Middle English potentat, from Old French, from Late Latin potentātus, from Latin, power, from potēns, present participle of posse, to be able; see potent.]
c.1400, from L.L. potentatus "a ruler," also "political power," from L. potentatus "power, dominion," from potentem (nom. potens) "powerful" (see potent).