Alternatively referred to as I/O (pronounced as eye-o), input/output is any software or hardware device designed to send and receive data to and from a computer hardware component. For example, a NIC (network interface card) is an I/O device because it sends and receives information from other network devices and computers. However, a computer mouse is only an input device, because it can send data but cannot receive any data back. A computer monitor is an output device because it can display information (output) but cannot send data back to the computer. Assuming it's not a touch screen.
With computer architecture, I/O is any device that sends and receives information directly from the CPU and memory. For example, reading from a hard drive is considered I/O because the hard drive is sending and receiving information from the CPU and memory.
See our I/O device page for further information about I/O devices and a list of I/O devices.