35 mm slides
From Just Solve the File Format Problem
35 mm slides came on film rolls of the same size and shape as 35 mm negatives, designed to fit the same cameras, but the pictures on them came out "positive" instead of "negative" so that they could be projected directly from the film onto a screen. Usually, after developing, the film is cut into separate pictures which are each mounted in cardboard so they can be inserted into slots of slide projectors (including carousels which can hold many slides for sequential projection).
For many years (from 1935 to 2009), Kodachrome was a very popular brand of color slide film.
