Webster's Dictionary 
(1): (n.) Anything resembling an amphitheater in form; as, a level surrounded by rising slopes or hills, or a rising gallery in a theater.
(2): (n.) An oval or circular building with rising tiers of seats about an open space called the arena.
Cyclopedia of Biblical, Theological and Ecclesiastical Literature 
a round or oval theatre, with tiers of seats, used chiefly by the Romans to exhibit the combats of gladiators or wild beasts. It was at least partially covered with awning. The general taste of that people for these amusements is proverbial, and they appear to have constructed amphitheatres at all their principal settlements. There are still considerable remains of them: in England the earthworks only exist at Cirencester, Silchester, and Dorchester; in France, much of the masonry exists at Arles, and at Nismes, in Languedoc; in Istria, at Pola; and in Italy, the well- known Colosseum at Rome; At Verona Capna, Pompeii, and many other places the buildings exist with their masonry very perfect. (See Theatre).