Actually by discofever, but included in the writeup The Ultraviolet Catastrophe
"an idealized object that absorbs and emits electromagnetic radiation on all frequencies. Most equations bank off of it; otherwise, it's too hard to get down the exact characteristics of how a material reacts to radiation into a formula. Might as well stick with an imaginary something that'll approximate any object, right? Anyway. " ...i realize that if The Ultraviolet Catastrophe is where you were coming from, this isn't much help - but what if you weren't? What then?