Lewis & Short

† hēmĭcyclium, ĭi, n., = ἡμικύκλιον,

  1. I. a half-circle, semicircle, Vitr. 9, 8, 5; Plin. Ep. 5, 6, 33.
  2. II. In partic.
    1. A. A semicircular recess, with seats, Cic. Lael. 1, 2; Sid. Ep. 2, 2.
    2. B. A semicircular public place furnished with rows of seats for learned discussions, Suet. Gramm. 17.
    3. C. A semicircular kind of sundial, Vitr. 9, 8, 1.