pentium 2? I think you're talking about a slot1 to socket 370 adapter where you could install a socket 370 cpu on top of a slot 1 card and would fit on a slot 1 mobo that are made for slot 1 cpu.
http://en.wikipedia.org/wiki/File:KL_Socket370_Slot1_Adapter.jpg
in that case, the thing that you put on top was called a cpu.
or
386 and before era, co-processor, it wasn't placed on top of the cpu but rather alongside on the mobo in a co-processor socket.
http://en.wikipedia.org/wiki/File:80386with387.JPG