From what i get from what i found it has to do with heat sink testing. Its a model of a cpu made out of copper or some other material. I guess it would be cheaper to use this than an actualy cpu when performing heat tests, since it would be fried by the heat. I think the term comes from die cast molding. I guess they make them different sizes since processors come in different sizes.
http://www.overclockers.com/tips263/index02.asp (first hit on google. although i should stop saying that or people will stop asking questions on the board.)