The name literally translates into “imperfect bone formation.”  This is a group of genetic disorders that target the stability of someone’s bones. People living with this condition are said to have a connective tissue disorder with a deficiency or mutation in one or more collagen gene, (specifically COL1A1 and COL1A2}. They have bones that break often and easily; usually from mild trauma or sometimes with no cause at all.  There are at least eight recognized forms, all with their own signs and symptoms.

The Journal of Neurology, Neurosurgery, and Psychiatry published an article (J Neurol Neurosurg Psychiatry 1999;66:357-364 doi:10.1136/jnnp.66.3.357) naming, “Basilar Impression as the commonest of the craniocervical malformations,” and stating it to be, “the upward displacement of basilar and condylar portions of the occipital bone causing an infolding of the foramen magnum and leading to a translocation of the upper cervical spine into the brainstem.” 

• http://jnnp.bmj.com/content/66/3/357.full