The word pentimento is derived from the Italian 'pentirsi', which means to repent or change your mind. Pentimento is a change made by the artist during the process of painting.
These changes are usually hidden beneath a subsequent paint layer. In some instances they become visible because the paint layer above has become transparent with time. Pentimenti (the plural) can also be detected using infra-red reflectograms and X-rays. They are interesting because they show the development of the artist's design, and sometimes are helpful in attributing paintings to particular artists.