hemicycle

1. A semicircle.
2. A semicircular structure or arrangement.

[French hémicycle, from Latin hēmicyclium, from Greek hēmikuklion : hēmi-, hemi- + kuklion, neuter of kuklios, circular (from kuklos, circle; see cycle).]

1. A semicircular arena.
2. A room or division of a room in the form of a semicircle.
3. A semicircular recess.

The noun has one meaning:

Meaning #1: a plane figure with the shape of half a circle
  Synonym: semicircle