Although philosophers and critics are happy to use the term "aesthetics", few have read the short treatise in which that term was coined. The book is Alexander Baumgarten's Reflections on Poetry -- actually Meditationes Philosophicae de Nonnulis ad Poema Pertinentibus, written in modern Latin and first published in 1735 (Baumgarten expanded upon the topic in his Aesthetica of 1750-8, which as far as I know has not yet been translated into any modern vernacular). In this book, Baumgarten, one of the later rationalists, asserted in quasi-Euclidean fashion a cumulative series of definitions and derivations about what he called "philosophical poetry". His definition of aesthetics comes in the antepenultimate and penultimate sections of the Reflections on Poetry, rendered as follows in the 1954 translation by Karl Aschenbrenner and William B. Holther (University of California Press, pp. 77-78):
§115. Philosophical poetics is (by §9) the science guiding sensate discourse to perfection; and since in speaking we have those representations which we communicate, philosophical poetics presupposes in the poet a lower cognitive faculty. It would now be the task of logic in its broader sense to guide this faculty in the sensate cognition of things, but he who knows the state of our logic will not be unaware how uncultivated this field is. What then? If logic by its very definition should be restricted to the rather narrow limits to which it is as a matter of fact confined, would it not count as the science of knowing things philosophically, that is, as the science for the direction for the higher cognitive faculty in apprehending the truth? Well, then. Philosophers might still find occasion, not without ample reward, to inquire also into those devices by which they might improve the lower faculties of knowing, and sharpen them, and apply them more happily for the benefit of the whole world. Since psychology affords sound principles, we have no doubt that there could be available a science which might direct the lower cognitive faculty in knowing things sensately.
§116. As our definition is at hand, a precise designation can easily be devised. The Greek philosophers and the Church fathers have already carefully distinguished between things perceived [ αισθητα ] and things known [ νοητα ]. It is entirely evident that they did not equate things known with things of sense, since they honored with this name things also removed from sense (therefore, images). Therefore, things known are to be known by the superior faculty as the object of logic; things perceived are to be known by the inferior faculty, as the object of the science of perception, or aesthetic [ aestheticae ].
Baumgarten held that aesthetics is not primarily the philosophy of art, of the beautiful, or of the sublime; rather, it is a psychological science that would do as much to clarify perceptual knowledge as logic had done to clarify conceptual knowledge. Granted, it is arguable how much logic has truly contributed to the clarification of human concepts (personally I think we are more indebted to the agonistic pursuits of scientists than to the armchair theorizing of philosophers and logicians); but I find it interesting that Baumgarten strongly associated aesthetic with the task of "knowing things sensately" and thus granted it an epistemological stature and importance that is too often missing from analyses of the arts.