1) A full-sized, preparatory drawing for a painting.  Cartoons were transferred to a prepared surface, such as a canvas or plastered wall, using the technique of pouncing.  Video here. Web resource here.

Raphael. Cartoon for the School of Athens (det.). Ca. 1510.  Charcoal and white lead on paper. Biblioteca Ambrosiana, Milan.

2) A typically simplified and/or exaggerated drawing intended for satire, caricature, or humor.

Cartoon by Mike Twohy. 2001.