CPU
A CPU (short for central processing unit) is the processor (usually a microprocessor or microcontroller) that's doing the main work of co-ordinating a system's overall operational flow.
This includes not just the microprocessor itself, but also things like its clock, if it doesn't have its own built in. In this sense, it's more the abstract concept of what's doing a particular task, rather than a particular discrete chip.
It's quite common for a single system to have multiple processors performing different tasks simultaneously. The designation of one of them as the CPU is purely based on what it's doing, not the resources it contains with which to do it.
For example, the pinball machine Firepower has two 6802 or 6808 microprocessors in it, one handling the main operational flow, and the other handling just the sound. Even though the chips are identical, only the former is regarded as its CPU.
It's even possible to have other processors, such as a GPU (graphics processing unit), that are more powerful than the main CPU itself.
As an analogy, if you think of the whole system as like an organisation, then the CPU is the boss while the other processors are the workers that report to them. The boss doesn't necessarily have to be faster or have more memory than the workers. It entirely depends on the organisation's structure, size, and the work it's performing.
Electronic components: CPU | Microcontroller | Microprocessor
