The hippocampus is a part of the forebrain, located in the medial temporal lobe. It belongs to the limbic system and plays major roles in long term memory and spatial navigation. Humans and other mammals have two hippocampi, one in each side of the brain. In rodents, where it has been studied most extensively, the hippocampus is shaped something like a banana. In humans, it has a curved and convoluted shape that reminded early anatomists of a seahorse. The name, in fact, derives from the Greek word for seahorse (Greek: hippos = horse, campos = sea).

In Alzheimer's disease, the hippocampus is one of the first regions of the brain to suffer damage; memory problems and disorientation appear among the first symptoms. Damage to the hippocampus can also result from oxygen starvation (anoxia), encephalitis or mesial temporal lobe epilepsy. People with extensive hippocampal damage may experience amnesia, that is, inability to form or retain new memories.