Bulging of tissue through an opening in a membrane, muscle or bone.

A protrusion of an organ or part through connective tissue or through a wall of the cavity in which it is normally enclosed — called also rupture.
A brain herniation is the displacement of brain tissue, cerebrospinal fluid, and blood vessels outside their normal structures in the head. A brain herniation can occur through a natural opening at the base of the skull (known as the foramen occipitalis) or through surgical openings. Herniation can occur between compartments inside the skull, such as those separated by a rigid membrane called the “tentorium”. A brain herniation occurs when pressure inside the skull (intracranial pressure) increases and displaces brain tissues. This is commonly the result of brain swelling from a head injury, or space-occupying lesions such as primary brain tumor, metastatic brain tumor, and hemorrhages or strokes that produce swelling within the brain. Hydrocephalus (accumulation of fluid in the brain) can also lead to brain herniation. A brain herniation often causes a massive stroke. This results from poor blood supply to some areas of the brain and compression of vital structures that regulate your breathing and circulation. This can rapidly lead to death or brain death.