Originally, corrections or additions made by an artist as final adjustments to a complete picture. However, the term has come to identify something quite different: the work done by a restorer to replace areas of loss or damage in a painting. In the past, retouches often covered broad areas of the original painting; contemporary conservation ethics dictate that retouching (or inpainting) be confined to the areas of loss. Retouches are executed in a medium that differs from the original so that they can be removed easily; oil paintings, for example, can be retouched with materials such as watercolors or synthetic resin-based paints.

Retouching is most often intended to remain invisible to the naked eye. Examination of a painting under ultraviolet light will usually reveal the presence of retouching; photographs are used by conservators to document paintings in their unretouched (or cleaned) state, and these also provide a clear record of areas that have been subsequently retouched.

Source: Looking at Paintings: A Guide to Technical Terms / eds. Tiarna Doherty and Anne T Woollett. (Los Angeles: J. Paul Getty Museum, 2009), p. 62.