A holocaust is a religious animal sacrifice that is completely consumed by fire. The word derives from the Ancient Greek holokaustos (ὁλόκαυστος from ὅλος "whole" and καυστός "burnt", with rough breathing), which is used solely for one of the major forms of sacrifice.
Holokautein (ὁλοκαυτεῖν) was one of the two chief verbs of Greek sacrifice, in which the victim is utterly destroyed and burnt up, as opposed to thyesthai (θύεσθαι), to share a meal with the god and one's fellow worshippers, commensal sacrifice. In the latter, the edible parts of the sacrificed animal were roasted and distributed for festive celebration, whereas the inedible parts were burned on the altar, those being the god's share. Although not actually obliged to do so, Greeks would rather sacrifice a domestic animal to a god or hero and then proceed to use its flesh as food, as animals were thought of as sharing in the sanctity of life - in addition to their secular usefulness (milk, eggs, ploughing). This did not apply to game, fish, and other seafood, which formed a far larger proportion of the diet then than they do today - fish was the major foodstuff sold in ancient Greek marketplaces. A sacrifice need not be a public function involving priests and altars; they could also be held privately, domestically or individually.
These are the two ideal types of Greek sacrificial ritual; they are appropriate to different divinities, done for different purposes, and conducted by different methods. Holocausts are apotropaic rituals, intended to appease the spirits of the Underworld, including the Greek heroes, who are spirits of the dead; they are also given to malign powers, such as the Keres and Hecate. One of the earliest attested holocausts was Xenophon's offering of pigs to Zeus Meilichius.
Holocausts are conducted at night, without wine, and offer black-hided animals at a low altar, with their heads directed downwards; in all these they are opposed to the commensal sacrifice given to the Olympian gods. (This distinction is between extreme types, and was somewhat exaggerated in the early twentieth century, as by Jane Harrison; considerable evidence has been also been found of commensal sacrifice offered to heroes.)