How does one define "Art"?

How exactly does one define art? Is art only that which is pleasing to the eye? I believe a hard, solid, well-made knife blade or a swift, crippling blow to the legs, is much more beautiful than a worthless, soft gold-plated blade or a graceful dance that does nothing than excite or relax? So is art in the eye of the beholder or is there a specific definition that cannot be changed?