The word chthonic (ˈθɒnɪk), or chthonian, is derived from the Ancient Greek word χθών, "khthon", meaning earth or soil. It translates more directly from χθόνιος or "in, under, or beneath the earth" which can be differentiated from Γῆ, or "ge", which speaks to the living surface of land on the earth. In Greek, chthonic is a descriptive word for things relating to the underworld and can be used in the context of chthonic gods, chthonic rituals, chthonic cults, and more. This is as compared to the more commonly referred-to Olympic gods and their associated rites and cults. Olympic gods are understood to reference that which exists above the earth, particularly in the sky. Gods that are related to agriculture are also considered to have chthonic associations as planting and growing takes place in part under the earth.
Chthonic and ouranic, or olympic, are not completely opposite descriptors. They do not cleanly differentiate types of gods and worship into distinct categories, but represent a cultic spectrum. These terms communicate associations with the underworld and/or agriculture. This makes some deities such as Hades, Persephone, and Erinyes more likely to be considered chthonic due to their proximity to the underworld. While this is the case, virtually any god could be considered chthonic to emphasize different aspects of the god. For example, Demeter and Hermes are categorized within the twelve Olympian gods but are often considered chthonic. Zeus has also been referenced with the surname "chthonios", demonstrating the situational use of a chthonic description.
In Ancient Greece, the names of deities were sometimes followed by an epithet. These epithets most often followed the name of the god, acting like a surname.
In this context, the purpose of an epithet was to describe a characteristic or association of a deity. The epithets 'chthonios' and 'chthonia' would follow the name of a god or goddess to reference their relationship either to the underworld or agriculture. For example, Hermes Chthonios references Hermes' role as the underworld escort.