What is meant exactly with 'no drift' on the 3D orientation output?

The orientation algorithm that Xsens has developed for its MotionTrackers is employing earth's gravity and magnetic field as a global reference. This is also known as an Attitude and Heading Reference System (AHRS) inertial measurement unit. By combining information of the different internal sensors drift, which would typically occur from integrating the angular rate of turn data, is eliminated.