The Collaborative International Dictionary of English v.0.48:

Hemicycle \Hem"i*cy`cle\, n. [L. hemicyclus, Gr. ?; ? +
   ky`klos.]
   1. A half circle; a semicircle.
      [1913 Webster]

   2. A semicircular place, as a semicircular arena, or room, or
      part of a room.
      [1913 Webster]

            The collections will be displayed in the hemicycle
            of the central pavilion.              --London
                                                  Academy.
      [1913 Webster]

WordNet (r) 3.0 (2006):

hemicycle
    n 1: a plane figure with the shape of half a circle [syn:
         semicircle, hemicycle]