herniation
 [her″ne-a´shun] abnormal protrusion of an organ or other body structure through a defect or natural opening in a covering membrane, muscle, or bone. (See also 
hernia.)

Herniation syndromes. From Ignatavicius and Workman, 2002.
central herniation a downward shift of the brainstem and the diencephalon due to a supratentorial lesion, causing Cheyne-Stokes respirations with pinpoint nonreactive pupils.
cingulate herniation a shift of the cingulate gyrus to below the falx cerebri.
disk herniation (
herniation of intervertebral disk) (
herniation of nucleus pulposus) 
herniated disk. tonsillar herniation protrusion of the cerebellar tonsils through the foramen magnum.
transtentorial herniation downward displacement of medial brain structures through the tentorial notch by a supratentorial mass, exerting pressure on the underlying structures, including the brainstem; this is a life-threatening situation because of pressure on the third cranial nerve, with symptoms including dilated, nonreactive pupils, ptosis, and a decreased level of consciousness. Called also 
caudal transtentorial herniation, 
tentorial herniation, and 
uncal herniation.
 Miller-Keane Encyclopedia and Dictionary of Medicine, Nursing, and Allied Health, Seventh Edition. © 2003 by Saunders, an imprint of Elsevier, Inc. All rights reserved.