Homer (Il. 15.184–93) recounts that when the cosmos was divided among the gods, Poseidon received the sea as his lot. Yet his first worshipers probably did not live within sight of the sea. Poseidon was a powerful god among the Mycenaean Greeks, and his cult is strongest among populations established in the Greek world before the so-called Dorian invasion. His status was gradually eroded in the Archaic period, as the process of Panhellenization required that all the gods of the canon be subordinated to Zeus. Little concerned with the spheres of justice, invention, or the arts, Poseidon is in origin a god of elemental, geological forces: life-giving springs, disastrous floods, chasms through which water flows or recedes, and tremors in the earth. Ultimately he ruled the vast and unpredictable sea, causing storms and tidal waves.
He is often found partnered with Demeter, a clue to his probable origin as a deity of fresh water. The most commonly cited etymology of his name recognizes it as a compound: Greek posis or potis, “lord, spouse” is combined with an element of unknown meaning, possibly “earth.”1 Poseidon’s name, then, contains the masculine version of the word potnia, or mistress, which is familiar from the Linear B tablets, while he himself appears in the tablets from Knossos and especially Pylos. One of his most widespread cult epithets, Asphaleios (Steadfast), was apparently a euphemism, emphasizing his power to still earthquakes rather than induce them. In both poetry and cult he is Ennosigaios (Earth-Shaker) and Gaieochos (Embracer of Earth). This control over the forces in the earth only occasionally spilled over into agricultural or chthonic, underworld functions, as at Tainaron, where Poseidon hosted an oracle of the dead.
Poseidon was also a god around whom many Greeks shaped their ethnic identities. For the Thessalians, the Boiotians, the people of Trozen, and many others, he was an ancestor, comparable to Zeus in the large number of heroes he sired with mortal maidens. Poseidon was an important amphictyonic deity, which means that his cult was the focus for many federations and leagues, whose shared interests were based sometimes on tribal affinity and sometimes on geographical proximity. According to the Homeric Hymn in his honor (22.5), Poseidon is “a tamer of horses and a savior of ships.” Myth made him the father of the horses Areion and Pegasos, while he was honored in many places as Hippios and was a master of chariot races from earliest times. He was the central deity at the Panhellenic sanctuary of Isthmia, worshiped with his consort Amphitrite.