Extent of herniation
It is the downward displacement of the cerebellum, specifically a region called the "tonsils", through the foramen magnum. There are actually 4 types.
Type one is only tonsillar herniation, that is mild; type ii shows more herniation, to include a middle portion of the cerebellum. This type is often associated with an myelomeningocele (herniation of lining of the brain).