What is a thematic development within the context of a fugue called?

In the context of a fugue, a thematic development is often referred to as an "episode." An episode serves as a contrasting section that provides a transition between entries of the subject or main theme and allows for the development of certain ideas within the music.

Episodes typically contain material derived from the theme, but they do not present the complete theme in its entirety, allowing the composer to explore related themes or motives. This helps to create variety and maintains listener interest while still drawing from the original thematic material.

While terms like “stretto,” “countersubject,” and “exposition” relate to specific aspects of fugue structure, they do not accurately describe the idea of thematic development occurring between statements of the main subject, which is the focus of this question. Stretto refers to overlapping entries of the subject, a countersubject is a secondary theme appearing in conjunction with the main subject, and the exposition is the initial presentation of the themes. An episode, however, is specifically designed for thematic exploration within a fugue’s overall structure.