In sonata form, the exposition is "the first major section, incorporating at least one important modulation to the dominant or other secondary key and presenting the principal thematic material."[2]
In a fugue, the exposition is "the statement of the subject in imitation by the several voices; especially the first such statement, with which the fugue begins."[3]
^Benward & Saker (2009). Music in Theory and Practice: Volume II, p.136-38. ISBN978-0-07-310188-0.