A good definition from Charlotte Mason:
As for that aesthetic 'appetency' (to use Coleridge's word) upon which so many of the gentle pleasures of life depend, it is open to many disasters: it dies of inanition when beauty is not duly presented to it, beauty in words, in pictures and music, in tree and flower and sky. The function of the sense of beauty is to open a paradise of pleasure for us; but what if we grow up admiring the wrong things, or, what is morally worse, arrogant in the belief that it is only we and our kind who are able to appreciate and distinguish beauty? It is no small part of education to have seen much beauty, to recognize it when we see it, and to keep ourselves humble in its presence.
vol 6 pg 56