I've been thinking about this and I've come up with a cross media definition that I like. An artist is the outcome of the sum of their influences and experiences. That stands true for grafitti artists, painters, makers of video installations, cartoonists, game designers, mathematicians, musicians etc etc. For any artist an enormous number of influences work on their creativity, a painter may begin by appreciating and being influenced by the works of Titian and seek to emulate it, he may then be introduced to paintings by Mark Rothko and find the use of colour will completely change the way he approaches his work. That same artist may then hear Miles Davis for the first time and his approach will modify again. Eventually the sheer quantity of influences, large and small, will be passed through the lense of the artists experience in the world to create something unique to them. Influences may still be visible, or may simply lead them to produce something that resembles anothers work in some way that suggests a school of thought but in some respect the work will be unique.
Without life experience to shape everything the art may be hollow however.