In anatomy,  the axis (from Latin axis, "axle") or epistropheus is the second cervical vertebra (C2) of the spine, immediately inferior to the atlas, upon which the head rests. The axis defining feature is its strong odontoid process (bony protrusion) known as the dens, which rises dorsally from the rest of the bone. Structure
The body is deeper in front or in the back and is prolonged downward anteriorly to overlap the upper and front part of the third vertebra. It presents a median longitudinal ridge in front, separating two lateral depressions for the attachment of the longus colli muscles. Dens
The dens, also called the odontoid process or the peg, is the most pronounced projecting feature of the axis. The dens exhibits a slight constriction where it joins the main body of the vertebra. The condition where the dens is separated from the body of the axis is called os odontoideum and may cause nerve and circulation compression syndrome. On its anterior surface is an oval or nearly circular facet for articulation with that on the anterior arch of the atlas. On the back of the neck, and frequently extending on to its lateral surfaces, is a shallow groove for the transverse atlantal ligament which retains the process in position. The apex is pointed and gives attachment to the apical odontoid ligament. Below the apex, the process is somewhat enlarged and presents on either side a rough impression for the attachment of the alar ligament; these ligaments connect the process to the occipital bone.
This type can be stable or unstable and may require surgery. [1]

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References
This article incorporates text in the public domain from page 99 of the 20th edition of Grays Anatomy (1918)

External links
Netter, Frank. Atlas of Human Anatomy Archived 2017-11-20 at the Wayback Machine, "High Cervical Spine: C1-C2"