A linkage between two things. A user interface is the system of controls with which the user controls a device. Two devices are said to be interfaced when their operations are linked electronically. An interface box is often required to convert signals from one form to another. For example, in order to get MIDI data in and out of a computer, you need some type of MIDI interface hardware. This may hook to an existing port on the computer, such as the printer port, or (in the case of the IBM-PC) it may consist of a circuit board that is plugged into one of the computer's internal slots.