What do you call the principal theme of a fugue?

The principal theme of a fugue is known as the subject. In the context of a fugue, the subject serves as the foundational melodic material that is presented at the beginning and subsequently developed throughout the piece. This theme is introduced in one voice and then imitated by other voices in a structured manner, which is characteristic of fugue writing. The interplay of the subject and its various entries and transformations is what gives a fugue its complexity and richness.

While "answer" refers to the subsequent statement of the subject in another voice after its initial presentation, and "counterpoint" signifies the relationship between two or more melodic lines, neither captures the essence of the primary theme itself. "Theme" is a more general term that can apply to various forms of music and does not specifically denote the unique role that the subject plays in a fugue. Thus, the subject is the correct term for the principal theme in this contrapuntal form.