In anatomy, the axis (from Latin axis, "axle") is the second cervical vertebra (C2) of the spine, immediately inferior to the atlas, upon which the head rests. The spinal cord passes through the axis.

The defining feature of the axis is its strong bony protrusion known as the dens, which rises from the superior aspect of the bone.

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.

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.

The internal structure of the odontoid process is more compact than that of the body. The odontoid peg is the ascension of the atlas fused to the ascension of the axis. The peg has an articular facet at its front and forms part of a joint with the anterior arch of the atlas. It is a non-weight bearing joint. The alar ligaments, together with the apical ligaments, are attached from the sloping upper edge of the odontoid peg to the margins of the foramen magnum. The inner ligaments limit rotation of the head and are very strong. The weak apical ligament lies in front of the upper longitudinal bone of the cruciform ligament and joins the apex of the deltoid peg to the anterior margin of the foramen magnum. It is the fibrous remnant of the notochord.

The pedicles are broad and strong, especially in the front, where they coalesce with the sides of the body and the root of the odontoid process. They are covered above by the superior articular surfaces.

The laminae are thick and strong. They play a large role in the stability of the cervical spine alongside the laminae of C7.