Well, assuming you are talking in terms of computers, it stands for Central Processing Unit (CPU). It's the little chip(s) on the inside of the computer that does exactly that, processes. It does the calculations and everything the computer does.
The Central Processing Unit is the main chip on the motherboard of a computer that controls all the other components that are attached to the motherboard. The chip is removable and if your motherboard permits it, you can swap it with a faster chip. The CPU can easily overheat and fry. For this reason, the chips have very large cooling fans attached to them. The chips can often be overclocked (if your motherboard permits it) but this will DEFINITELY overheat the CPU. To overclock a computer, you need to buy special cooling fans to prevent overheating. A lot of people use the word CPU to refer to the entire computer but this is wrong. A CPU refers only to a chip that will fit in the palm of your hand.