Lewis & Short

sīpărĭum, ii, n. [kindr. with supparum, from σίπαρος, orig. a little sail; hence, transf.],

  1. I. The smaller curtain in a theatre (drawn up between the scenes of a comedy; cf. aulaeum, the main curtain): aulaeo subducto et complicitis sipariis, scena disponitur, App. M. 10, p. 253, 23; 1, p. 106, 4: post siparium, i. e. behind the scenes, Cic. Prov. Cons. 6, 14; Juv. 8, 186.
      1. 2. Meton., comedy (opp. cothurnus, tragedy): (verba) cothurno, non tantum sipario fortiora, Sen. Tranq. 11, 6.
  2. II. A curtain or screen over the judges’ seats to keep off the sun, Quint. 6, 1, 32 Spald.; 6, 3, 72.