What defines an episode in a fugue?

In the context of a fugue, an episode is specifically defined as a passage that occurs between statements of the main theme or subject. Unlike the main entries of the subject, which are characterized by a clear and identifiable thematic material, an episode typically lacks this identifiable subject or answer. It serves to create contrast and variety within the fugue by allowing for the exploration of new musical ideas or fragments derived from the main theme without fully restating it.

Episodes can modulate to different keys and provide a sense of development and movement. They are crucial for maintaining interest and building tension in the overall structure of the fugue, allowing the composer to transition smoothly back into statements of the subject. Thus, the defining characteristic of an episode is its nature of not featuring the original subject or its answer, focusing instead on development and variation while sustaining the overall cohesion of the piece.