The Collaborative International Dictionary of English v.0.48:

AEsthetics \[AE]s*thet"ics\, Esthetics \Es*thet"ics\ (?; 277),
   n. [Gr. ? perceptive, esp. by feeling, fr. ? to perceive,
   feel: cf. G. [aum]sthetik, F. esth['e]tique.]
   The theory or philosophy of taste; the science of the
   beautiful in nature and art; esp. that which treats of the
   expression and embodiment of beauty by art.
   [1913 Webster]

WordNet (r) 3.0 (2006):

aesthetics
    n 1: (art) the branch of philosophy dealing with beauty and
         taste (emphasizing the evaluative criteria that are applied
         to art); "traditional aesthetics assumed the existence of
         universal and timeless criteria of artistic value" [syn:
         aesthetics, esthetics]

Moby Thesaurus II by Grady Ward, 1.0:

38 Moby Thesaurus words for "aesthetics":
   artistic taste, axiology, casuistry, connoisseurship, cosmology,
   dilettantism, epicureanism, epicurism, epistemology, ethics,
   expertise, expertism, first philosophy, friandise, gastronomy,
   gnosiology, logic, mental philosophy, metaphysics,
   moral philosophy, ontology, phenomenology, philosophastry,
   philosophic doctrine, philosophic system, philosophic theory,
   philosophical inquiry, philosophical speculation, philosophy,
   school of philosophy, school of thought, science of being,
   sophistry, theory of beauty, theory of knowledge, value theory,
   virtu, virtuosity