The American Heritage® Dictionary of the English Language, 4th Edition
n. An underlying image in a painting, as an earlier painting, part of a painting, or original draft, that shows through, usually when the top layer of paint has become transparent with age.
n. An alteration in a painting, evidenced by traces of previous work, showing that the artist has changed his or her mind as to the composition during the process of painting.
WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.
n. the reappearance in a painting of an underlying image that had been painted over (usually when the later painting becomes transparent with age)
Word Usage
"That is called pentimento because the painter 'repented', changed his mind."