Webster's Dictionary 
(1): ( n.) The act of exposing or laying open; a setting out or displaying to public view.
(2): ( n.) Situation or position with reference to direction of view or accessibility to influence of sun, wind, etc.; exposure; as, an easterly exposition; an exposition to the sun.
(3): ( n.) A public exhibition or show, as of industrial and artistic productions; as, the Paris Exposition of 1878.
(4): ( n.) The act of expounding or of laying open the sense or meaning of an author, or a passage; explanation; interpretation; the sense put upon a passage; a law, or the like, by an interpreter; hence, a work containing explanations or interpretations; a commentary.
Cyclopedia of Biblical, Theological and Ecclesiastical Literature 
"the opening up and interpreting larger portions of Scripture in public discourses. In Scotland, where the practice has long obtained, and still extensively prevails, it is called lecturing. While the striking and insulated texts of Scripture, which furnish abundant matter for sermons, are calculated, when judiciously treated, to rouse and fix attention; and the discourses founded on them may be more useful to general hearers, especially the careless and unconverted, expository discourses furnish peculiar advantages as it regards the enlargement of the Christian's views of divine truth, and his consequent advancement in the ways of God. By judiciously expounding the Scriptures, a minister may hope to give a clearer exhibition of the great principles of religion in their mutual connections and diversified bearings than could otherwise be done. He will have a better opportunity of unfolding the true meaning of those parts of the Bible which are difficult of bringing a vast variety of topics before his hearers, which may be of the utmost importance to them, but which he could not so conveniently have treated in preaching from detached texts of exhibiting the doctrines and duties of Christianity in their relative positions of successfully counteracting and arresting the progress of dangerous errors, and of storing the minds of his people with correct and influential views of divine things. (See Doddridge on Preaching.) Such a mode of public instruction cannot but prove of great use to a minister's own mind, by rousing his energies, habituating him to close and accurate research, and saving him much of that indecision in the choice of texts which is so much lamented" (Buck, Theolog. Dictionary, s.v.). Dr. James W. Alexander was very earnest in advising expository preaching. "It is the most obvious and natural way of conveying to the hearers the import of the sacred volume. It is the very work (to interpret the Scriptures) for which the ministry was instituted." He advises exposition of whole chapters or books in course, pleading for it not only the sanction of ancient usage, but also certain great advantages of the method both to the preacher and his hearers (Thoughts on Preaching, N.Y. 1867, 12mo, page 272 sq.). (See Homiletics).