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?