What is "nekyia"

In ancient Greek cult-practice and literature, a nekyia is a "rite by which ghosts were called up and questioned about the future," i.e., necromancy. A nekyia is not necessarily the same thing as a katabasis. While they both afford the opportunity to converse with the dead, only a katabasis is the actual, physical journey to the underworld undertaken by several heroes in Greek and Roman myth.

In common parlance, however, the term "nekyia" is often used to subsume both types of event, so that by Late Antiquity for example " Olympiodorus ... claimed that three [Platonic] myths were classified as nekyia (an underworld story, as in Homer's Odyssey book 11)".