In a computer, the CPU is the small part that does the real work of following instructions — adding numbers, deciding what happens next, running programs. The letters stand for central processing unit. People who like computers talk about it a lot, and shops print CPU on boxes and ads. Though actually, a lot of people point at the whole tall box under the desk and call that the CPU, when that box also holds the memory, the power supply, and other pieces. The careful name for that box is the case, or just the computer. Phones and tablets have a CPU too, but in everyday talk people more often say processor or chip. It is a technical word, but a very common one, and it is a little more formal than chip.