Lewis Short
hēmĭcyclium (noun N) = ἡμικύκλιον:
* A half-circle, semicircle, Vitr. 9, 8, 5; Plin. Ep. 5, 6, 33.
* In partic.
* A semicircular recess, with seats, Cic. Lael. 1, 2; Sid. Ep. 2, 2.
* A semicircular public place furnished with rows of seats for learned discussions, Suet. Gramm. 17.
* A semicircular kind of sundial, Vitr. 9, 8, 1.
Charlton T. Lewis, Charles Short, A Latin Dictionary