A congenital or aquired, cranio-cervical junction abnormality where the tip of the 2nd cervical (axis, odontoid process) vertebra is displaced upward and posteriorly (backward) into the intracranial space projecting above the foramen magnum. Basilar Impression is caused by this deformation resulting from softening of the bone (as opposed to Basilar Invagnation wherein the bone is normal).  Basilar Invagnation and Basilar Impression both can cause potential compression of the brainstem.