amphitheater

noun3

1. •
   An open, outdoor theatre (which may be a theatre in the round, or have a stage with seating on only one side), especially one from the classical period of ancient Greece or Rome, or a modern venue of similar design.

   • Ancient Roman amphitheatres were mostly oval or circular in plan, with seating tiers that surrounded the central performance area, like a modern open-air stadium.
2. •
   American form of amphitheatre.

   • The symphony warmed up inside the amphitheater while the audience crowded around outside.
3. •
   A natural formation of a similar shape, where a steep mountain or slope or particular rock formation forms a partial or complete bowl, especially one used as a performance space (and possibly modified by carving out seats, etc) because the slopes naturally amplify or echo sound.

   • Brecon or Aberhonddu, a typical Welsh town of 5,350 inhabitants, stands in a grand amphitheatre of hills and mountains at the confluence of the rivers Usk, Honddu, and Tarell. — 1939