(or tight rough, or tie down)
A clean rough is a rough drawing that has been completely fleshed out without being cleaned up on a separate sheet of paper. This is usually done when the animator is satisfied with the action of the scene, and is ready to pass the scene off to an assistant or cleanup artist. It serves the purpose of providing definite, clear, on model keys to the assisting and ensures the preservation of the qualities of the rough in the final cleaned up drawings.
Here's an example:
Open these two drawings in a separate window and flip between them so you can see how closely related the rough is to the tied down version.
By maintaining the line work and construction of the rough while combining it with the final line of the finished drawing, you can show how you intend your drawings to be cleaned up. Now when the clean up artist has to deal with a drawing that hasn't been tightened up, they can use the tight rough as a guide for how to translate the rough structure into solid cleanups. It's a way to tell everyone that has to deal with your drawings what you really mean by your roughs. It's also your last chance to correct anything that hasn't been dealt with yet. Volumes, eye direction, structure problems... anything that needs to be pushed or clarified do it now, because if it doesn't get dealt with now, it won't be fixed down the line.