No discussion about one of the major Hellenic Theoi would be complete without a discussion of his or her epithets and titles. Poseidon is no exception to this.
It is no surprise that many of his titles are connected with areas of Hellas as there is literally no part of modern Greece that is over 100 miles (160 km) away from the sea. These titles include: Ισθμιος (Isthmios) — referring to the area around Corinth, Ταιναριος (Tainarios) — referring to the area of Sparta, Γενεσιος (Genesios) — referring to Argolis, Σαμιος (Samios) — Of Samos, Αιγαιος (Aigaios) — referring to the area of Aegae in Euboea, and both Ἑλικωνιος (Helikônios) and Ονχηστιος (Onkhêstios) referring respectively to the areas of Helicon and Onchestus in Boetia. Looking at this, it can easily be inferred that Poseidon’s cultus was wide-spread and almost universal throughout Hellas.
He had the title of Βασιλευς (Basileus) — meaning King — due to his being the ruler of the oceans. Πελαγαιος (Pelagaios) — of the Sea — and Αιγαιων (Aigaiôn) — of the Aegean — are direct references to his relationship to the oceans. His title of Προσκλυστιος (Prosklystios) — who dashes against — refers to the action of the oceans on the shore, both the tides and the occasional flooding due to storms. He is referred to as Ασφαλιος (Asphalios) in reference to his role in ensuring safe voyages. All of these, are because of the importance of the oceans in the life of Hellas.
He is both Γαιηοχος (Gaiêokhos) — the Holder of the Earth, and Εννοσιγαιος (Ennosigaios) — the Earth-Shaker. Interestingly, in Classical mythology, the oceans both supported and surrounded the land so Poseidon would certainly be connected to the Earth in this way. Because of his role as the creator and nurturer of horses, Poseidon has the epithets of Ἱππιος (Hippios) — of the Horses — and Ἱπποκουριος (Hippokourios) — Horse Tender.
There are also references to him being Πατρος (Patros) — Father — and Λαοιτης (Laoitês) — of the People. These seem to refer to him being considered an ancestral and protective deity. As this is a common theme in cult centers for many of the Hellenic Theoi, I would not be surprised if the cultus paid to demigod heros such as Bellerophon and Theseus as founders and ancestors of various city-states extended to Poseidon in this way.