MDI stands for Medium Dependent Interface. It is the physical and electrical connector on a network interface card or switch port that links directly to the cabling, such as an RJ-45 port for twisted-pair Ethernet. The wiring pinout is standardized so that devices can transmit and receive signals correctly across the physical medium.
Heard on the job
If you're connecting two switches without auto-MDIX, you'll need a crossover cable instead of a straight patch.