Sabattier effect
An effect created in a wet darkroom in which an image is processed so that it is partially a positive image as it would normally appear and partially a negative image. It wasn't until the 1860s that it was first characterized, but Man Ray's work is what brought it to widespread attention (1890-1976). During the process of developing a print, his assistant, Lee Miller (1907-1977), inadvertently turned on a light. Although Ray loved the effect, he did not intentionally employ it in his work. He referred to it as "solarization." The Sabattier effect can be reproduced with relative ease in Photoshop, and it is recommended that it be applied to a monochrome photograph.
Do you have a better definition? Share it in the comments.