Lewis & Short

exōstra, ae, f., = ἐξώστρα.

  1. I. A machine in the theatre, by which the back part of the stage was turned towards the spectators; transf. of any thing public: quibuscum jam in exostra helluatur, Cic. Prov. Cons. 6, 14.
  2. II. A hanging bridge, applied by besiegers to the walls of a besieged city, Veg. Mil. 4, 21.