epeisodion

from The Century Dictionary.

• noun In the ancient Greek drama, especially in tragedy, a part of a play following upon the first entrance (the parodos) of the chorus, or upon the entrance or reëntrance of actors after a stasimon or song of the whole chorus from its place in the orchestra; hence, one of the main divisions of the action in a drama; a division of a play answering approximately to an act in the modern drama.