CPU is a silicon integrated circuit, sometimes called a computer
chip. This is because it consists of a square piece
of crystallized silicon that is very, very thin.
It has a very
intricate pattern on top, which consists of the transistors and
wires that have been processed into the silicon: a silicon chip
contains millions of tiny transistors connected by very tiny wires.
The chip is also connected to the rest of the computer with very
tiny strands of wire. Here is a photograph of the Intel Pentium 4 chip:
for larger view.