Complex sentence: Modern usage for "amphitheater" is lax, and does not always respect the ancient usage, and so the word can be found describing theatre-style stages with the audience only on 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.