An artist’s alteration (literally, repentance or change of mind). While usually used to refer to a modification that has become increasingly apparent with time, the term may also describe a change that is visible only through scientific analysis. As an artist develops a painting, she or he may choose to alter the placement of particular elements with the composition. Although these changes are initially invisible on the finished surface of the painting, they can become visible over time, emerging as ghostlike images. This is the result of the fact that paint (in particular, oil paint) tends to become more transparent as it ages; changes lying below the surface will become more evident as the transparency increases.

Source: Looking at Paintings: A Guide to Technical Terms / eds. Tiarna Doherty and Anne T Woollett. (Los Angeles: J. Paul Getty Museum, 2009), p. 55.