Final answer:
The opening section of a fugue where each voice introduces the main theme is called the exposition, during which the subject of the fugue is also established.
Step-by-step explanation:
The opening section of a fugue, in which all voices successively introduce the subject, is called the exposition. In this section, each voice enters at different times with the same theme, establishing the harmonic framework of the piece and setting the stage for the interplay of the various parts.
The subject of a fugue is the main theme, the primary melodic idea that is introduced at the beginning of the fugue and is imitated by all the parts throughout. Following the exposition, the fugue may enter a developmental phase known as the episode, where the composer can introduce new material or creatively manipulate the subject.