An outline drawing serving as a guide to printing each of the colours of a print.
A drawing representing any of the key points in an animation (e.g. a sound effect, the beginning or end of a particular movement, etc.) between which intermediate frames are interpolated in order to create a continuous sequence.
Mid 19th century; earliest use found in Once a Week: an illustrated miscellany of literature, art, science, and popular information. From key + drawing.