A clear plastic sheet onto which a drawing is copied, either by hand-inking or by a xerographic copier process. Colored paints are applied to the reverse side. One or more cels may be placed over a painted background, which serves as a setting for the action. In animated movies and cartoons, twenty-four cels are required for each second of screen time. (That's 86,400 per hour, folks!)
Cel is an abbreviation for Celluloid™.