I have wanted to post on this thread all day and have struggled with how to articulate myself (I apologise if I don't actually make any sense)! I love the definition Angie has provided, however, being a musician, my own personal definition of beauty is closely linked to how I define joy.
Simply, to me, beauty is something or someone that makes my heart sing. I consider beauty can sometimes be represented by sleepy lullaby sung by a tired mother to her newborn child; it could be a fast and heavy rock-song sung at a stadium to a sold out crowd; or a passionate and virtuosic opera singer performing a stunning aria - all very different genres but all equally capable of creating that same special feeling inside.
Edited to say.... Hmmmmm. Have just re-read that and have realised whilst true for me, it is very corny. Sorry about that everyone!