As an abstract artist I am often asked this question. It is valid regardless of how inept I might be at formulating an answer. I believe this is because there are so many answers to this question.
For example, 'Art,' as I would use it is defined as "the expression or application of human creative skill and imagination, typically in a visual form such as painting or sculpture, producing works to be appreciated primarily for their beauty or emotional power." It is also, defined as, "the various branches of creative activity, such as painting, music, literature, and dance. The visual arts" Then you have, "Subjects of study primarily concerned with the processes and products of human creativity and social life, such as languages, literature, and history (as contrasted with scientific or technical subjects) or the belief that the arts and sciences were incompatible." Finally, it is also defined as, "a skill at doing a specified thing, typically one acquired through practice such as the art of conversation, or even the art of war."
So then it would seem to myself and other artists that art is something we do, a verb. It has been stated and written that, " Art is an expression of our thoughts, emotions, intuitions, and desires, but it is even more personal than that: it’s about sharing the way we experience the world, which for many is an extension of personality. It is the communication of intimate concepts that cannot be faithfully portrayed by words alone. And because words alone are not enough, we must find some other vehicle to carry our intent. But the content that we instill on or in our chosen media is not in itself the art. Art is to be found in how the media is used, the way in which the content is expressed."
I think an excellent example of this would be some of the works of Mark Rothko.
As you can see in Rothko's "Orange and Yellow (1956), the edges of the rectangles are never distinct, avoiding an optical break and allowing viewers' eyes to move quietly from one area to another in a contemplative way. Rothko did not want us to think about him when looking at his paintings, so he tried to remove all evidence of the creation process. To accomplish this, he applied numerous layers of thin paint with a brush or rag to unprepared canvas, which absorbed the colors into its fabric. The many thin washes help to give his paintings a lightness and brightness, as if they glow from within. Orange and Yellow was considered quite large in the 1950s, and Rothko asked viewers to stand close in order to be visually surrounded by the colors. His goal was for color to, in his words: "Express basic human emotions - tragedy, ecstasy, doom. The people who weep before my pictures are having the same religious experience I had when I painted them. And if you, as you say, are moved only by their color relationships, then you miss the point."
The truth, my friends is that all works of art may elicit a sense of wonder or cynicism, hope or despair, adoration or spite. The art itself can be direct or complex, subtle or explicit, intelligible or obscure. This means that an artist's subjects and their approaches to the creation of art are bounded only by the imagination of the artist. It is because of this that I believe that defining art based upon its content is inherently wrong.
Alistair MacFarlane once wrote that, "Art is a way of grasping the world. Not merely the physical world, which is what science attempts to do; but the whole world, and specifically, the human world, the world of society and spiritual experience."
I cannot speak for all of you, but I am prone to agree with Mr. MacFarlane.
I hope that you enjoyed this and that I didn't bore you completely.