Harry Thurston Peck, Harpers Dictionary of Classical Antiquities (1898)

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Absis

or Apsis (ἁψίς). Literally, a fastening of any kind. It was applied specially to the joining together of the extremities of a piece of wood, so as to give it the shape of a bow; and hence it came to signify anything of that shape, such as a bow, an arch, or a wheel (Hes. Op. 424; Herod.iv. 72.) The next transition of meaning is to anything vaulted (for example, ἡ ὑπουρανία ἁψίς, the vault of heaven, Phaedr. 247 B); and in this sense it was adopted in architecture, first, for any building or portion of a building of a circular form, or vaulted (Epist. ii. 17.18), and more especially for the circular and vaulted end of a basilica (Paul. Nol. Ep. 12). In Christian churches the apse came to mean the end of the choir, where the bishop's throne was placed.