Mobile PCI Express Module
Mobile PCI Express Module (MXM) is an interconnect standard for GPUs (MXM Graphics Modules) in laptops using PCI Express created by MXM-SIG. The goal was to create a non-proprietary, industry standard socket, so one could easily upgrade the graphics processor in a laptop, without having to buy a whole new system or relying on proprietary vendor upgrades. == Generations == Smaller graphics modules can be inserted into larger slots, but type I and II heatsinks will not fit type III and above or vice versa.
Source: Wikipedia — Mobile PCI Express Module (CC BY-SA 4.0)