The Collaborative International Dictionary of English v.0.48:

Apse \Apse\ ([a^]ps), n.; pl. Apses ([a^]p"s[e^]z). [See
   Apsis.]
   1. (Arch.)
      (a) A projecting part of a building, esp. of a church,
          having in the plan a polygonal or semicircular
          termination, and, most often, projecting from the east
          end. In early churches the Eastern apse was occupied
          by seats for the bishop and clergy. Hence:
      (b) The bishop's seat or throne, in ancient churches.
          [1913 Webster]

   2. A reliquary, or case in which the relics of saints were
      kept.
      [1913 Webster]

   Note: This word is also written apsis and absis.
         [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Absis \Ab"sis\, n.
   See Apsis.
   [1913 Webster]