Hypocephalus (Gr.). A kind of a pillow for the head of the mummy. They are of various kinds, e.g., of stone, wood, etc., and very often of circular disks of linen covered with cement, and inscribed with magic figures and letters. They are called “rest for the dead” in the Ritual, and every mummy-coffin has one.
Source: H.P.Blavatsky - The Theosophical Glossary