--The quality of a communication.--
People are capable of near infinite concepts and ideas. Writing, painting, singing, talking, movies and many more are all means by witch you can convey your ideas to another. But did they get it? How clearly do your words get across a 3D object? If you drew a picture of a celebrity would people that see it recognize the celeb?
Art is simply how well you can use the tools and techniques of any medium to get that medium to communicate a thought, person, action, feeling, you name it.
I once saw a painting that I was told was done by a pro. When I looked at it, I nor anyone could tell what the point was of the painting. Thus he was not a pro at all.
"In the movie, you see the battle field and right away you know this is going to be epic. The music starts slowly and your heart starts pounding. You see the ferocity on faces and feel the desperation these people had. The weapons shatter under steel and muscle. It was only then that I realized what it must have been like and what these people had to go through to win their freedom." Now that is art.