Alzheimer's disease

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

• noun A degenerative disease of the brain, seen primarily in elderly people and associated with the development of abnormal tissues and protein deposits in the cerebral cortex, and characterized by disorientation, memory failure, speech disturbances, and the progressive loss of mental capacity.

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

• noun neurology, pathology A disorder involving loss of mental functions resulting from brain tissue changes; senile dementia of Alzheimer's type.

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

• noun a progressive form of presenile dementia that is similar to senile dementia except that it usually starts in the 40s or 50s; first symptoms are impaired memory which is followed by impaired thought and speech and finally complete helplessness