The development of multiple cognitive deficits manifested by both:

memory impairment (impaired ability to learn new information or to recall previously learned information)
one (or more) of the following cognitive disturbances:
aphasia (language disturbance)
apraxia (impaired ability to carry out motor activities despite intact motor function)
agnosia (failure to recognize or identify objects despite intact sensory function)
disturbance in executive functioning (i.e., planning, organizing, sequencing, abstracting)
The cognitive deficits above each cause significant impairment in social or occupational functioning and represent a significant decline from a previous level of functioning. The course is characterized by gradual onset and continuing cognitive decline. The deficits do not occur exclusively during the course of a delirium.

The cognitive deficits above are not due to any of the following:

other central nervous system conditions that cause progressive deficits in memory and cognition (e.g., cerebrovascular disease, Parkinson's disease, Huntington's disease, subdural hematoma, normal-pressure hydrocephalus, brain tumor)
systemic conditions that are known to cause dementia (e.g., hypothyroidism, vitamin B-12 or folic acid deficiency, niacin deficiency, hypercalcemia, neurosyphilis, HIV infection)
substance-induced conditions