A slide is a “positive” piece of film mounted in a holder (paper, plastic, sometime metal frame with glass), and are designed to be put in a projector for display on a screen.

When purchasing 35mm film years ago, you could purchase it 2 ways, negative film designed to make photographic prints, or slide film, designed for projection with a projector. You could always print a slide, but it was more costly than printing a negative, due to the difference in photographic papers. I believe, most professional photographers would use slide film, and then look at the slides on a light board and a magnifying glass to determine which slides they wanted to use for say the magazine or newspaper. Many non professional’s would also buy slide film and have a family evening watching slide shows that dad took.