Definition: A cel is the transparent sheet of plastic upon which final animation artwork is transferred in order to be painted in and then laid over a background and filmed.
The line art of each frame of animation is either inked or copied onto the front of the cel, and then paint applied to the back. When the transparent cels are laid over the static backgrounds, they form a complete picture without having to redraw the same background for each frame of animation.
Early cel animation was done on nitrate cels, which are not only prone to decay, but quite flammable. Currently acetate cels, which are much less volatile and more durable, are an industry standard.
