In mythology chthonic (from Greek χθονιος-pertaining to the earth; earthy) designates, or pertains to, gods or spirits of the underworld, especially in Greek mythology.
Greek khthon is one of several words for "earth"; it typically refers to the interior of the soil, rather than the living surface of the land (as does gaia or ge) or the land as territory (as does khora). It evokes at once abundance and the grave.
Its pronunciation is somewhat awkward for English-speakers — for this reason, many American dictionaries recommend that the initial "ch" should be silent. However, most reputable dictionaries, such as the OED, state that the first two letters should be pronounced "k".
Chthonian and Olympian
While terms like "Earth deity" have rather sweeping implications in English, the words khthonie and khthonios had a more precise and technical meaning in Greek, referring primarily to the manner of offering sacrifices to the god in question.
In the typical chthonic cult, the animal victim was slaughtered into a bothros "pit" or megaron "sunken chamber". In the cult of the Olympian gods, by contrast, the victim was sacrificed onto a raised bomos "altar". Chthonic deities also tended to favor black victims over white ones, and their offerings were usually burned whole or buried rather than being cooked and shared among the worshippers.
H.P. Lovecraft applied the term 'Chthonian' for a fictional specie in his Cthulhu mythos.
Cult type versus function
While chthonic gods and goddesses had a general association with fertility, they did not have a monopoly on it, nor were Olympian gods wholly unconcerned from the earth's prosperity. Thus Demeter and Persephone both watched over aspects of the fertility of land, yet Demeter had a typically Olympian cult while Persephone had a chthonic one.
Even more confusingly, Demeter was worshipped alongside of Persephone with identical rites, and yet was occasionally classified as an "Olympian" in poetry and myth.
The categories Olympian and Chthonic were not, however, hard and fast. Some Olympian gods, like Hermes and Zeus, also received chthonic sacrifices and tithes in certain locations. The deified heroes Heracles and Asclepius might be worshipped as gods or chthonic heroes, depending on the site.
Moreover, a few deities are not easily classifiable under these terms. Hecate, for instance, was typically offered puppies at crossroads — not an Olympian sacrifice, to be sure, but not a typical offering to Persephone or the heroes, either. But because of her underworld functions, Hecate is generally classed as chthonic.