Wikipedia
M-Modules are a mezzanine standard mainly used in industrial computers. Being mezzanines, they are always plugged on a carrier PCB that supports this format. The modules communicate with their carrier over a dedicated bus, and can have all kinds of special functions.
M-Modules are standardized as ANSI/VITA 12-1996 and are especially suited for adding any kind of real-world I/O to a system in a flexible way. They are modular I/O extensions for all types of industrial computers, from embedded systems up to high-end workstations. The M-Module Interface - a fast asynchronous parallel interface - offers sophisticated functions like 32-bit data bus, burst transfers up to 100 MB/s, DMA and trigger capabilities. M-Modules also offer direct front-panel connection rather than requiring a separate adapter panel with ribbon-cable connections. This provides a clean path for sensitive signals without loss of data or signal quality - using, for example, shielded D-Sub connectors and coaxial cables.