1. An acronym for Input/Output. This is also typically used to describe the parts of a computer that get data into the computer or get data out of the computer. For example, a keyboard is an input device, however, you can't detect that data is coming back out of the computer, even though it may trigger the return of data. A printer is an output device. A monitor is also an output device but with the advent of touch screens, can also be an input device. Serial ports are usually the gateways for devices that are both input and output, such as a modem. As technology has changes, parallel ports are now 2-way (bi-directional (Bi-Di) as well. The term BIOS is an acronym for Basic Input Output System.
2. A description of a port in a computer's memory scheme. A location in a separate memory address map maintained for CPU/device communication. Each device's I/O ports are uniquely assigned within the map. I/O ports come in sets of 2, 4, 8, 16 and 32, and are referred to only by their Base Address. Often used in PC's in conjunction with memory-mapped I/O, where I/O Ports form the control channel, and memory access supplies the data channel. Except for PCs and the backward compatibility issues, the technology has been replaced by memory-mapped I/O.