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