1) Niche 2) Of or relating an apse 3) Of or relating to an apse 4) Recess 
shaped like an apse
• (a.) Of or pertaining to the apsides of an orbit. • (a.) Of or pertaining to the apse of a church; as, the apsidal termination of the chancel.
Ap'si·dal (ăp'sĭ*d a l) adjective 1. (Astron.) Of or pertaining to the apsides of an orbit. 2. (Architecture) Of or pertaining to the apse of a church; as, the apsidal termination of the chancel. 
In architecture, the term apsidal refers to something pertaining to the apse of a church; for example the apsidal termination of the chancel.
[adj] - of or relating an apse
adjective of or relating to an apse