CP/M is an operating system developed by Digital Research, Inc., originally released in 1974.
The ZX Spectrum +3 included CP/M support in ROM. The version included is CP/M Plus, which provides compatibility for programs written for the older CP/M 2.2, matching that provided by the Amstrad CPC.
CP/M was used by Sinclair Research for the development of the ZX Spectrum 128, in the form of a Z80 expansion card  capable of supporting two instances of CP/M, attached to the corporate VAX-11/780. If two instances were in use, a CP/M system could be emulated using the VAX's own CPU, but used a great deal of processing power, which was discouraged, as by the mid-1980s, the same VAX was used heavily within Sinclair Research as their corporate computing environment, including email. The Microsoft M80 macro-assembler and L80 linker were used to assemble and link the ZX Spectrum 128's ROM image.