What does it mean by word size in computer?

"Word size" refers to the number of bits processed by a computer's CPU in one go (these days, typically 32 bits or 64 bits). Data bus size, instruction size, address size are usually multiples of the word size.

Just to confuse matters, for backwards compatibility, Microsoft Windows API defines a WORD as being 16 bits, a DWORD as 32 bits and a QWORD as 64 bits, regardless of the processor.

Consider the following:

CPU (processor), RAM (main memory), I/O devices (mouse, keyboard, printer), Bus (data transfer component).

How would you like these computer parts to communicate and transfer data? you definitely need a fixed size of bits to be considered a single unit of data.

For that, Computer scientists agreed to standardize this unit to be 32 bits or 64 bits (depending on the manufacturer choice).

They gave this unit a name and called it a Word.

So a Word is nothing but a unit of data (bunch of bits (signal charges of zeros and ones)) that moves around from a computer component to another.

For instance buses are built with 32 bits (4 bytes) and some with 64 bits (8 bytes). Likewise with the CPU (hardware) and operating systems (software) are built with either 32 bits or 64 bits.

It just happened to be the standard unit named Word and sized 32 bits or 64 bits.

Ps: Word is one of the many data size units that move around inside the computer, different computer components use different sizes to transport data (signals charges that represent zeros and ones), for instance RAM can use size of 64 bits while Buses can use 32 bits. Hardware designers design the architecture of components taking into account these size differences to either implement Word size of 32 bits on only CPU but 64 bits on RAM, or implement the same size on all components, ...etc. Word size used to be 8 bits (1 byte), but nowadays the most comment unit size is 64 bits on most computer components such as CPU or RAM, or Bus, ...etc.

One answer is -- not as much as it used to. Way back when, computers could only load/store full "words" of memory, which would be 16/24/32/36/48 bits each (depending on the particular machine). One would have to carefully structure their program design around the word size of the target machine.

But any more computers can access individual bytes or can access strings of data dozens of bytes long, with one relatively seamless operation. Much more significant than the technical word size is the memory buss width, which determines how many bytes can be transferred between CPU and memory in one memory "cycle".

So "word size" is somewhat meaningless, and (as another answer suggests) companies like Microsoft will often define it in an arbitrary manner, with no real relationship to anything.