In photography, a slide is a thin translucent photograph bound in paper, plastic or similar frame and often used for projection with use of a slide projector for a slideshow. The terms reversal film and transparency are also used.
One can distinguish it from negative and print.
A single slide, showing a color transparency in a plastic frame
Italian reversal film from the 1950s: Ferraniacolor
Slide frames, 1940 (metal) to 1985 (plastic)
Slide viewer
Slide frame 6x6 cm
