Chthonic (pron.: /ˈkθɒnɪk/, from Greek χθόνιος – chthonios, "in, under, or beneath the earth", from χθών – chthōn "earth"; pertaining to the Earth; earthy; subterranean) designates, or pertains to, deities or spirits of the underworld, especially in relation to Greek religion. The Greek word khthon is one of several for "earth"; it typically refers to the interior of the soil, rather than the living surface of the land (as Gaia or Ge does) or the land as territory (as khora (χώρα) does). It evokes at once abundance and the grave.

The pronunciation is somewhat awkward for English speakers. Most dictionaries, such as the OED, state that the first two letters should be pronounced as [k], /ˈkθɒnɪk/; others, such as the AHD, record these letters as silent, /ˈθɒnɪk/. The modern pronunciation of the Greek word "χθόνιος" is [xθoɲos], although the Classical Greek pronunciation would have been something similar to [kʰtʰonios].