MDI Vs. MDIX
The terminology generally refers to variants of the Ethernet over twisted pair technology that use a female 8P8C port connection on a computer, or other network device.
The X refers to the fact that transmit wires on an MDI device must be connected to receive wires on an MDIX device. Straight through cables connect pins 1 and 2 (transmit) on an MDI device to pins 1 and 2 (receive) on an MDIX device. Similarly pins 3 and 6 are receive on an MDI device and transmit on an MDIX device. The general convention was for network hubs and switches to use the MDIX configuration, while all other nodes such as personal computers, workstations, servers and routers used an MDI interface. Some routers and other devices had an uplink/normal switch to go back and forth between MDI and MDIX on a specific port.
Read more about this topic: Medium Dependent Interface