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I/O port(1) (Input/Output port) An I/O port is a socket on a computer that a cable is plugged into. The port connects the CPU to a peripheral device via a hardware interface or to the network via a network interface. See port, standards - hardware interfaces, DisplayPort, HDMI and USB.
(2) (Input/Output port) In a PC, an I/O port is an address used to transfer data. See PC I/O addressing.