Solid Drawing

From Anipedia

Solid Drawing is one of the 12 Principles of Animation. The idea behind this principle is that every frame in an animation should be able to stand out on its own as a solid drawing. Sometimes inbetweens are treated as secondary drawings to key poses, often with the excuse that they may only be on screen for a 12th or 24th of a second, so their quality is negotiable. However, solid in-betweens are just as important as solid keys. Without solid drawing, animations can appear to morph or warble as they change in quality.

Solid Drawing has been achieved when you can pause an animation at any time and the frame can stand out on its own as an individual piece of still art.