Short for medium dependent interface crossover (the ?X? representing ?crossover?), an Ethernet port connection that allows networked end stations (i.e., PCs or workstations) to connect to each other using a null-modem, or crossover, cable. Commonly this is an extra connector on for example a router or switch which is crossed. Commonly this port is in parallel with the first or last port of the router.

Media Dependent Interface with Crossover (MDIX). A cable used for hubs and switches.