Vertebra, second cervical: The second cervical vertebra is called the axis. It is so-named because the uppermost cervical vertebra (called the atlas) rotates about the odontoid process of the second cervical vertebra. The joint between the axis and atlas is a pivot type of joint. It allows the head turn.
The Latin word "axis" means axle or pole. The axis bone serves as the axle about which the atlas (and the head) turn.