What term describes the original idea in a fugue?

The term that describes the original idea in a fugue is the subject. In the structure of a fugue, the subject is the primary melodic idea that is introduced at the beginning and serves as the foundational material upon which the entire composition is built. The subject is then subjected to various transformations and elaborations throughout the piece, which is characteristic of the intricate contrapuntal techniques associated with fugal writing.

In a fugue, the subject typically appears in all voices as it is developed, imitated, and transformed through techniques such as inversion, augmentation, and diminution. This makes the subject crucial for establishing the thematic coherence of the work.

In contrast, the other terms refer to different aspects of the fugue. The counter subject is a secondary melody that is introduced when the subject is stated, typically accompanying the subject in later entries. An episode is a passage that connects statements of the subject, often featuring material that may not be directly derived from the subject. Finally, while a theme can refer to a primary musical idea in broader musical contexts, in the specific structure of a fugue, the term subject is precisely defined and used.