Definition of Axis
Axis: The axis is the second cervical vertebra (symbol: C2). It is called the "axis" because the uppermost cervical vertebra (called the atlas) rotates about the odontoid process of C2. 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.