An electronic component that measures tilt and motion.
A device with an accelerometer knows what angle it is being held at. It can also measure movements such as rotation, and motion gestures such as swinging, shaking, and flicking.
One common use in phones it to detect whether the phone is upright or sideways, and automatically rotate the graphics on the screen accordingly.
Another common use is controlling games and other applications (such as music player) by moving or shaking the phone.