is Art the context where you found this word? The full sentence would help.
In the art context, specifically in the painting techniques, a pentimento is a change made by the painter to the picture while he is executing the painting itself, usually compared to the original sketch he might have done on the surface (be it a wall for a fresco or the canvas). The different course of the painting goes over the original sketch, which can sometimes be detected under the newer lay of paint by an X-ray test or other more advanced tast systems.
Studying the pentimenti can give an idea of the development of the painter work, his/her technique and rethinking of the details or the overall layout, and generally speaking are usually considered the proof that the painting is not a copy.