AIDS dementia complex: A brain disorder that occurs in people with AIDS that causes the loss of cognitive capacity, affecting the ability to function in a social or occupational setting. The exact mechanism by which HIV triggers the AIDS dementia complex has not been determined, but it may result from HIV infection of cells in the brain or from an inflammatory reaction to such an infection.

The AIDS dementia complex is an AIDS-defining illness: one of the serious illnesses that occurs in HIV-positive individuals and a reason for an AIDS diagnosis according to the definition of AIDS by the CDC (Centers for Disease Control & Prevention).