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 Gaia or Ge does) or the land as territory (as khora (χώρα) does). 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 other dictionaries, such as the OED, state that the first two letters should be pronounced as [k], /ˈkθɒnɪk/. Note that the modern pronunciation of the Greek word "χθόνιος" is [xθonios], although the Classical Greek pronunciation would have been something similar to [ktʰonios].
Some chthonic cults practised ritual sacrifice, which often happened at nighttime. When the sacrifice was a living creature, the animal was placed in a bothros ("pit") or megaron ("sunken chamber"). In some Greek chthonic cults, the animal was sacrificed on a raised bomos ("altar"). Offerings usually were burned whole or buried rather than being cooked and shared among the worshippers.
Not all Chthonic cults were Greek, nor did all cults practice ritual sacrifice; some performed sacrifices in effigy or burnt vegetable offerings.
While chthonic deities had a general association with fertility, they did not have a monopoly on it, nor were the later Olympian deities wholly unconcerned for 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 Persephone with identical rites, and yet occasionally was classified as an "Olympian" in late poetry and myth. The absorption of some earlier cults into the newer pantheon versus those that resisted being absorbed is suggested as providing the later myths that seem confusing however.
The categories Olympian and chthonic weren't, however, completely separate. Some Olympian deities, such as 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 and the time of origin of the myth.
Moreover, a few deities aren't easily classifiable under these terms. Hecate, for instance, was typically offered puppies at crossroads — not an Olympian type of sacrifice, to be sure, but not a typical offering to Persephone nor the heroes, either. Because of her underworld roles, Hecate is generally classed as chthonic.
In Jungian psychology, the term chthonic was often used to describe the spirit of nature within, the unconscious earthly impulses of the Self, one's material depths, but not necessarily with negative connotations.
For example: "Envy, lust, sensuality, deceit, and all known vices are the negative, 'dark' aspect of the unconscious, which can manifest itself in two ways. In the positive sense, it appears as a 'spirit of nature', creatively animating Man, things, and the world. It is the 'chthonic spirit' that has been mentioned so often in this chapter. In the negative sense, the unconscious (that same spirit) manifests itself as a spirit of evil, as a drive to destroy."
In Activision's Battlezone, the Cthonians were an advanced race of people who lived on Earth before humans. They populated many if not all planets in this solar system. There is also evidence in the expansion pack The Red Odyssey to suggest they spread far across the universe. Their psychical form has neither been shown or suggested throughout the entire series.
In ID Software's 1996 first-person shooter Quake, Chton is a boss enemy at the end of the game's first episode, Dimension of the Doomed. He appears in the form of a demonic entity which rises out of a pit of lava. Being immune to conventional weaponry, the boss character must be killed through the use of surreptitiously placed switches, which activate an electric current, killing the monster.
In Northern Lights, the universe which Lyra inhabits has a Cthonic Railway system as a parallel to the London Underground.