A part of an image that was deliberately painted over by the artist,  but shows through when the top paint layer becomes worn with age or is over-cleaned.  Pentimenti (pl.) can also be detected using infra-red reflectography and X-rays. Pentimenti usually occur when the painter decides to alter part of the image at a late stage in the painting.

Leonardo da Vinci. Salvator Mundi (detail of Christ’s right hand with pentimento). Oil on panel. Ca. 1499-1510. Location unknown.