Stands for "Medium Dependent Interface." MDI is a type of Ethernet port found on network devices. It is often used in contrast with MDIX (or "MDI-X"), which is similar to MDI, but switches the transmit and receive pins within the interface.
When connecting two devices with an Ethernet cable, it is important that the transmit pins on one end line up with the receive pins on the other. If you connect a device with an MDI port to a device with an MDIX port, a standard straight through Ethernet cable will do the job. However, when connecting two devices with MDI ports, such as two computers, an Ethernet crossover cable is required. The crossover cable switches the send and receive ports on the two connectors, allowing data to flow correctly between two MDI or MDIX ports.
On some network devices, the ports are labeled MDI or MDIX to help you choose the right type of cable. Routers often have multiple MDIX ports and a single MDI port called an "uplink" port designed to connect a device such as a cable modem. If you connect a device to the wrong port or use the wrong cable, it may not be able to communicate with the router.
Fortunately, Hewlett Packard developed a technology called Auto-MDIX (or "MDI/MDIX Auto Cross") that automatically switches between MDI and MDIX as needed. Today, most 10Base-T and 100Base-T network devices have Auto-MDIX ports, so it does not matter if you connect other devices using straight through or crossover cables. Additionally, Giagbit Ethernet (1000-BaseT) does not define specific send/receive pins, so it is not necessary to identify MDI or MDIX ports when connecting two Gigabit Ethernet devices.