Short for input/output, and pronounced eye-oh. I/O refers to any operation, program, or device whose purpose is to enter data into a computer or to extract data from a computer.
One usually uses the term I/O to distinguish noncomputational parts of a program from other parts that are strictly computational, or to distinguish certain devices from other devices. For example, a printer is an I/O device, whereas a CPU is a computational device.
All computer applications contain both I/O and computational parts. A word-processing system, for instance, contains I/O components (for entering, displaying, and printing text) as well as non-I/O components (for checking spelling, searching for words, and so on).