A small, pen-like instrument used to "write" on a touch-screen device.
A stylus provides far more accuracy than could be achieved with a fingertip, and provides a familiar pen-like experience for inputting handwriting or drawings.
A stylus is often nothing more than a simple stick of plastic, except that its tip is specifically designed to not damage the screen.
More advanced styli may have one or more buttons, and some include tip-pressure and/or motion sensors. Some include Bluetooth so they can double as a remote control.
See: Touch Screen