For my sins, I am a Fellow of the Royal Society for the Encouragement of Arts, Manufactures and Commerce, better known as the RSA. For the best part of three hundred years, this question has been one of its key preoccupations.
There are probably more answers than there have ever been members. I will suggest here only that art is subjective, management and commerce are (in principle) objective, and design is their marriage in an ever-evolving and sometimes turbulent union.