Derived from the Italian word for repentance, it is an art term used to describe the reapparance of an oilpainting or drawing that the artist (or a subsequent artist) overpainted. It can be a deliberate act, but it can also happen by the upper layers of paint turning gradually transparent through ageing, wear and tear.
Most museums have one or two examples of pentimento. Among them especially thinly painted panels by Dutch artist De Hooch, in which the floor painted before the figures suddenly emerge through the dresses and furniture added in the foreground. In other cases it is the revelation of previously completed figures which appear to be cloned simply because the artists have changed the position of one person in a group portrait to another location in the picture resulting in an identical and uintentional twin.
In contemporary art it is sometimes a result of the chemical composition of the various media added to the work of art which are not compatible with others and create unpredictable chemical results over time.
A good restorer can often fix the problem of pentimento quite easily when armed with todays science.