Wikipedia
Optical printer
An optical printer is a device consisting of one or more film projectors mechanically linked to a movie camera. It allows filmmakers to re- photograph one or more strips of film. The optical printer is used for making special effects for motion pictures, or for copying and restoring old film material.
Common optical effects include fade outs and fade ins, dissolves, slow motion, fast motion, and matte work. More complicated work can involve dozens of elements, all combined into a single scene.