If you run into someone you know well, and they look you right in the face but don't recognize you — perhaps they have prosopagnosia, or "face blindness." Prosopagnosia is a neurological condition characterized by the inability to recognize faces.
People who suffer from prosopagnosia can see someone's face, but they can't connect the face with the identity of the person they're seeing — even if the person is a family member or close friend. The condition may be genetic, or it may be acquired after a brain injury or disease. To compensate, people with prosopagnosia use other cues to identify people, such as a person's voice or distinctive clothing. The word prosopagnosia comes from the Greek words prosopon, meaning "face," and agnosia, meaning "not knowing."