Abstract Art is a work of art with intellectual or emotional meaning (or both) that does not represent or imitate any visible object or figure. Good abstract art is not easy to get to grips with - you may need to see a large number of works (in an exhibition devoted to one artist, for example) to begin to see and understand fully what she or he is trying to express. One example in a mixed show is often meaningless, because all you can hope to notice are the eye-catching or superficial qualities of the art.
Abstract Art link
Cumming, Robert (2001). Art: A Field Guide (p. 372). New York: Alfred A. Knopf