64-bit CPUs have existed in supercomputers since the 1960s and in RISC-based workstations and servers since the early 1990s. In 2003 they were introduced to the (previously 32-bit) mainstream personal computer arena, in the form of the x86-64 and 64-bit PowerPC processor architectures.

A CPU that is 64-bit internally might have external data buses or address buses with a different size, either larger or smaller; the term "64-bit" is often used to describe the size of these buses as well. For instance, many current machines with 32-bit processors use 64-bit buses (e.g. the original Pentium and later CPUs), and may occasionally be referred to as "64-bit" for this reason. Likewise, some 16-bit processors (for instance, the MC68000) were referred to as 16-/32-bit processors as they had 16-bit buses, but had some internal 32-bit capabilities. The term may also refer to the size of an instruction in the computer's instruction set or to any other item of data (e.g. 64-bit double-precision floating-point quantities are common). Without further qualification, "64-bit" computer architecture generally has integer registers that are 64 bits wide, which allows it to support (both internally and externally) 64-bit "chunks" of integer data.