brain damage

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Injury to the brain that is caused by various conditions, such as head trauma, inadequate oxygen supply, infection, or intracranial hemorrhage, and that may be associated with a behavioral or functional abnormality.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun injuries to the nervous system
• noun stupidity

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun injury to the brain that impairs its functions (especially permanently); can be caused by trauma to the head, infection, hemorrhage, inadequate oxygen, genetic abnormality, etc.