” He was the god of the Sea and other waters; of earthquakes; and of horses. In pre-Olympian Bronze Age Greece, he was venerated as a chief deity at Pylos and Thebes. Poseidon was protector of seafarers, and of many Hellenic cities and colonies. In Homer’s Iliad, Poseidon supports the Greeks against the Trojans during the Trojan War. In the Odyssey, during the sea-voyage from Troy back home to Ithaca, the Greek hero Odysseus provokes Poseidon’s fury by blinding his son the Cyclops Polyphemus, resulting in Poseidon punishing him with storms, the complete loss of his ship and companions, and a ten-year delay. Poseidon is also the subject of a Homeric hymn. In Plato’s Timaeus and Critias, the island of Atlantis was Poseidon’s domain. His Roman equivalent is Neptune.
The earliest attested occurrence of the name, written in Linear B, is Po-se-da-o or Po-se-da-wo-ne, which correspond to Ποσειδάων (Poseidaōn) and Ποσειδάϝονος (Poseidawonos) in Mycenean Greek; in Homeric Greek it appears as Ποσειδάων (Poseidaōn); in Aeolicas Ποτειδάων (Poteidaōn); and in Doric as Ποτειδάν (Poteidan), Ποτειδάων (Poteidaōn), and Ποτειδᾶς (Poteidas). The form Ποτειδάϝων(Poteidawon) appears in Corinth. A common epithet of Poseidon is Ἐνοσίχθων Enosichthon, “Earth-shaker”, an epithet which is also identified in Linear B, E-ne-si-da-o-ne, This recalls his later epithets Ennosidas and Ennosigaios indicating the chthonic nature of Poseidon.
The origins of the name “Poseidon” are unclear. One theory breaks it down into an element meaning “husband” or “lord” (Greek πόσις (posis), from PIE *pótis) and another element meaning “earth” (δᾶ (da), Doric for γῆ (gē)), producing something like lord or spouse of Da, i.e. of the earth; this would link him with Demeter, “Earth-mother”. Walter Burkert finds that “the second element da- remains hopelessly ambiguous” and finds a “husband of Earth” reading “quite impossible to prove”.
Another theory interprets the second element as related to the word *δᾶϝον dâwon, “water”; this would make *Posei-dawōn into the master of waters. There is also the possibility that the word has a Pre-Greek origin. Plato in his dialogue Cratylus gives two alternative etymologies: either the sea restrained Poseidon when walking as a “foot-bond” (ποσίδεσμον), or he “knew many things” https://en.wikipedia.org/wiki/Poseidon