Wikipedia
The Hetoimasia, Etimasia (Greek ἑτοιμασία, "preparation"), prepared throne, Preparation of the Throne, ready throne or Throne of the Second Coming is the Christian version of the symbolic subject of the empty throne found in the art of the ancient world, whose meaning has changed over the centuries. In Ancient Greece it represented Zeus, chief of the gods, and in early Buddhist art it represented the Buddha. In Early Christian art and Early Medieval art it is found in both the East and Western churches, and represents either Christ, or sometimes God the Father as part of the Trinity. In the Middle Byzantine period, from about 1000, it came to represent more specifically the throne prepared for the Second Coming of Christ, a meaning it has retained in Eastern Orthodox art to the present. The motif consists of an empty throne and various other symbolic objects, in later depictions surrounded when space allows by angels paying homage. It is usually placed centrally in schemes of composition, very often in a roundel, but typically is not the largest element in a scheme of decoration.