CPU
CPU stands for Central Processing Unit. It is basically the "brain" of a computer.
CPUs can have multiple cores. CPU cores today are typically 32-bit or 64-bit. For the sake of this article, we will refer to a single core as CPU. A CPU has multiple CPU registers. A register is a CPU internal storage, anywhere from just a single up to 32-bits or more.
CPU registers
Wordsize of a CPU
The Wordsize of a CPU is not really clearly defined. A true 8-bit CPU will have only 8-bit registers, with the exception of the PC, which needs to be wider in order to not limit the program memory to just 256 bytes. An example of a true 8-bit CPU would be the 6502. The 8080 or Z80 processors are also considered 8-bit CPUs, even though a lot of the registers can be grouped and actual arithmetic operations can be performed on them, such as in add HL, BC. Older CPUs were usually accumulator centered, so had some register that was used for most arithmetic operations. One way of telling the Wordsize of a CPU would then be the size of the accumulator. Note that the size of the bus interface has nothing to do with the Wordsize. The bus interface can even be 8-bit on a 32-bit CPU.
Thus we would define the wordsize as the number of bits processed by a typical instruction.
In Embedded Systems 8-bit and 16-bit CPUs are still widely used.
