Accelerometer

An accelerometer is a device that measures  acceleration forces such as static and dynamic. By measuring the static acceleration the accelerometer can tell the angle a device (such as the Wii remote) is tilted with respect to earth. It measures the dynamic acceleration to analize the way a device moves. So in the Wii remote an accelerometer is responsable for telling the Wii how the Wii remote is moving.

http://www.dimensionengineering.com/accelerometers.htm