Wikipedia
Multi-image is the now largely obsolete practice and business of using 35 mm slides ( diapositives) projected by single or multiple slide projectors onto one or more screens in synchronization with an audio voice-over or music track. Multi-image productions are also known as multi-image slide presentations, slide shows and diaporamas and are a specific form of multimedia or audio-visual production.
One of the hallmarks of multi-image was the use of the wide screen panorama. Precisely overlapping slides were placed in slide mounts with soft-edge density masks; when the resulting images were projected, the images would blend seamlessly on the screen to create the panorama. By cutting and dissolving between images in the projectors, animation effects were created in the panorama format.
The term multi-image is sometimes used to describe digital photo image computer programs that combine or change images on-screen, for photo montages, and image stitching.