component
In its most basic form, virtual reality systems consist of a computer and software—known as the reality engine—and input and output sensors. A sensor is a device that responds to some physical stimulus. A human eye, for example, is a sensor that responds to light rays.
In standard computer technology, input devices are the familiar keyboard, mouse, knobs, and joysticks; output devices include the printer and video display. Virtual reality input/output devices include head- and earmounted equipment for hearing and seeing and gloves for controlling the virtual world. The fourth "component" is the user, who directs the chosen environment and reacts to it.