Pluto, god of the nether world, son of Kronos and Rhea, brother of Zeus and Poseidon, and husband of Persephone; on the dethronement of Kronos the universe was divided among themselves by the three brothers, Zeus assuming the dominion of the upper world and Poseidon that of the ocean, leaving the nether kingdom to him, a domain over which and forth of which he ruled with a greater and more undisputed authority than the other two over heaven, earth, and sea.
Definition taken from The Nuttall Encyclopædia, edited by the Reverend James Wood (1907)
Plutarch * Plutonic Theory