a word
/ wurd /
A word is the natural gulp size of a particular processor — the number of bits it likes to handle in one go inside its working registers. If a byte is the smallest piece you can address, the word is the comfortable mouthful the CPU was built to chew. On the laptop or phone you are likely using, the word is 64 bits (eight bytes); a generation ago it was 32 bits (four bytes).
Concretely, the word size is usually how wide the CPU's general-purpose registers are and how big an address it can naturally hold. On a 64-bit machine a register such as rax holds 64 bits, and a memory address is a 64-bit number, so the CPU can in principle reach an enormous range of memory. When people call a processor '64-bit', that word size is what they are talking about — not the speed and not the amount of memory installed.
Be careful: 'word' is overloaded. The hardware word size (above) is a property of the CPU, but some software and file formats define their own fixed 'word' (for instance, the Windows API type WORD is always 16 bits regardless of the machine). In C the word size is not a type you write directly; instead the language gives you sized types and types like size_t and uintptr_t that are chosen to match the machine's natural width.
On a 64-bit CPU, one word = 8 bytes = 64 bits. A pointer fits in one word, and registers like rax, rsp, and rip are each 64 bits wide.
The word size is the CPU's natural register and address width.
'64-bit' describes the word size, not the speed or the RAM; and a software 'word' (like the 16-bit WORD type) can disagree with the hardware word, so always check which one is meant.