Complex sentence: Modern usage for "amphitheater" does not always respect ancient usages, and is sometimes used to describe theatre-style stages with spectator seating on only one side, theatres in the round, and stadiums.
Split into simpler sentences: Modern usage for "amphitheater" is lax, and does not always respect the ancient usage. As a result, the word can be found describing theatre-style stages with the audience only on one side, theatres in the round, and stadiums.