Definition: "Clean-Up" refers to the process of refining the rough artwork of 2D animation.
The purpose of clean-up is to recreate the animation with smooth, consistent line art and line weights before it is transferred to cels for painting.
Sometimes animators mark notes on the clean-up art in pencil or ink, in colors that won't transfer to the cells, on the cleaned line work - usually for the inkers and painters who'll be dealing with the various line and fill colors on the cels.
