Inbetween

Copy and paste from http://www.skaarj.com/gimpystick/faileas/440

An inbetween is when you take two frames of animation, lay them down on the light table so you can see them both, and on a clean sheet draw the pose inbetween them. An "inbetween" is usually very simple.

Inbetweens are straight numbers: 5. They're usually the most previlent types of frames

Example of two frames and their inbetween: