Degrees of freedom
Degrees of freedom or DOF is the ways an object can move within a space. There are a total of 6 degrees of freedom in a 3 dimensional space. The 6 DOF can be divided into 2 categories, rotational movements and translational movements. Each category has 3 DOF. Oculus Rift (Platform) and SteamVR HMDs all have 6 DOF. Both orientation tracking (rotation) and positional tracking (translation) are required to have a truly immersive VR experience.
The 3 rotational movements are pitch, yaw, and roll. These movements are tracked by most HMDs' on-board sensors. As you tilt and turn your head, the HMD senses the movements and alters its display accordingly.
Rotational Movements are tracked by IMUs or inertial measurement units consist of accelerometer, gyroscope and magnetometer. These IMUs measure the HMD's velocity, orientation, and gravitational forces to infer rotational orientation and movement. The 3 IMUs are often marketed as having "9 DOF". The 9 DOF are calculated by adding up the 3 DOF detected by each IMU. In reality, accelerometer, gyroscope and magnetometer all measure the same 3 DOF: pitch, yaw, and roll.
The 3 translational movements are left/right, forward/backward and up/down. These movements are usually tracked by an external camera or other sensors. Few HMDs use on-board sensors to track translational movements.
The ability to tracking translational movements are required for positional tracking, the ability to determine the absolute position of an object in a 3D environment.