Photographic film which, when developed, forms a positive image on the film itself. In the case of 135 (35mm film), each frame is mounted in a 2" square plastic or card mount for use in a projector.
As the film only has to go through development, and not printing (as for print film), there is less opportunity for the developers to mess up the colour; but the exposure time is much more critical as mistakes cannot be rectified in printing. Printing is possible by either an internegative (print film photo of the slide) or direct, by use of a positive print paper such as Cibachrome.
Slide film tends to be used by enthusiasts and professionals, due to the extra accuracy required, and the more elaborate display procedure (slide viewer or projector).
Popular films: Kodachrome 25/64/200, Velvia (50 ASA). Update: Kodachrome 25 is no longer available, and will be sorely missed by me.
Also known as transparency film; a slide is often referred to as a tranny.