One of the more accurate and widely accepted definitions of dementia (of which Alzheimer's is one type) was produced by a working party of doctors and neuroscientists in the US in 1984.
It goes; 'Dementia is the decline in the memory and other cognitive functions in comparison with the patient's previous level of function as determined by a history of decline in performance and by abnormalities noted from clinical examination and neuropsychological tests'.
A diagnosis of dementia cannot be made when consciousness is impaired or when other clinical abnormalities prevent other adequate evaluation of mental state. Dementia is a diagnosis based on behavior and cannot be determined by brain scan, EEG or other laboratory instruments, although specific causes of dementia may be identified by these means.'
NINCDS-ADRDA (National Institute of Neurological and Communicative Diseases and Stroke/Alzheimer's Disease and Related Disorders Association) work group, 1984