The odontoid process is an upward projection of bone that arises from the front part of the center of the axis vertebra. The axis is the 2nd highest spinal bone. The atlas is the first (topmost) bone in your neck and it sits on top of the axis.
Unlike most other spinal vertebrae, the atlas does not have a vertebral body. It is shaped like a ring through which the odontoid process passes. This arrangement allows for a lot of freedom of motion between the combination of the head and first vertebra, and the 2nd vertebra (axis).
The odontoid process provides a pivot point - called an axis of motion - around which the skull and the first cervical vertebra (the atlas) rotate and turn. Other cervical levels also participate in head turning and tilting.
The odontoid process is also called the dens.
Related: Parts of the Spinal Bone - Illustrations and Diagrams
Ishii T, Mukai Y, Hosono N, Sakaura H, Fujii R, Nakajima Y, Tamura S, Sugamoto K, Yoshikawa H. Kinematics of the subaxial cervical spine in rotation in vivo three-dimensional analysis. Spine (Phila Pa 1976). 2004 Dec 15;29(24):2826-31.