key drawing
/ˈkiː ˌdrɔː(r)ɪŋ/noun
1 Printing and Printmaking. An outline drawing serving as a guide to printing each of the colours of a print.
2 Film. 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.
Origin
Mid 19th century; earliest use found in Once a Week: an illustrated miscellany of literature, art, science, and popular information. From key + drawing.