• microprocessor: indicates the width of the registers.
• graphics device, such as a scanner or digital camera
• operating system: refers primarily to the number of bits used to represent memory addresses.

See also 32-bit and "What Is 64-bit Computing?" in Webopedia's Did You Know... section.

What Is 64-bit Computing?
When reading about PCs and servers, you'll often see the CPU described by the number of bits (e.g., 32-bit or 64-bit), here's a little info about what that means.