Coefficient of Restitution.

You can think of it as the force with which the coverstock is compressed and then rebounds with an XX force or unit of work. The USBC has a limit on the number; I have no idea what it is supposed to be.

Thank you!  I couldn't think of it as I haven't heard that term in a long time. Would thickness of a shell have a lot to do with this?  I would think a thicker shell would 'hit' harder than a shell that is thin.

Probably not. I think it's a combination of both the coverstock and the filler material's basic chemical and physical properties.

Some people thought the old Lane Masters balls, which were basically no filler balls, hit harder because they had no filler (just coverstock and core). I think it was because the cores were made of urethane. Anyway, when Lane Masters came out with their first ball with filler, the Sure Strike, they said they went to particular lengths to insure that the filler had good restitution qualities, testing many variations. Personally I think the Sure Strike hit harder than many of their 2 piece balls. I'm not sure what makes the best hitting balls.

The new Roto-Grip Hysteria seems to have no filler (and its ancestors, the Nomads, also had no fillers). Will they/did they hit harder???

I agree best scores I've ever had are with no filler balls 800 with a Solid Nomad, 300's with a Legends Yeah Baby, other best scores with multiple no filler Visionary balls.  Since, my best scores always seem to be no filler balls I may have to re-think my arsenal.