The hippocampus (pl.: hippocampi; via Latin from Greek ἱππόκαμπος, 'seahorse'), also hippocampus proper, is a major component of the brain of humans and many other vertebrates. In the human brain the hippocampus, the dentate gyrus, and the subiculum are components of the hippocampal formation located in the limbic system. The hippocampus plays important roles in the consolidation of information from short-term memory to long-term memory, and in spatial memory that enables navigation. In humans and other primates the hippocampus is located in the archicortex, one of the three regions of allocortex, in each hemisphere with direct neural projections to, and reciprocal indirect projections from the neocortex. The hippocampus, as the medial pallium, is a structure found in all vertebrates.

In Alzheimer's disease (and other forms of dementia), the hippocampus is one of the first regions of the brain to be damaged; short-term memory loss and disorientation are included among the early symptoms. Damage to the hippocampus can also result from oxygen starvation (hypoxia), encephalitis, or medial temporal lobe epilepsy. People with extensive, bilateral hippocampal damage may experience anterograde amnesia: the inability to form and retain new memories.

Since different neuronal cell types are neatly organized into layers in the hippocampus, it has frequently been used as a model system for studying neurophysiology. The form of neural plasticity known as long-term potentiation (LTP) was initially discovered to occur in the hippocampus and has often been studied in this structure. LTP is widely believed to be one of the main neural mechanisms by which memories are stored in the brain.

Using rodents as model organisms, the hippocampus has been studied extensively as part of a brain system responsible for spatial memory and navigation. Many neurons in the rat and mouse hippocampi respond as place cells: that is, they fire bursts of action potentials when the animal passes through a specific part of its environment. Hippocampal place cells interact extensively with head direction cells, whose activity acts as an inertial compass, and conjecturally with grid cells in the neighboring entorhinal cortex.

The earliest description of the ridge running along the floor of the inferior horn of the lateral ventricle comes from the Venetian anatomist Julius Caesar Aranzi (1587), who likened it first to a silkworm and then to a seahorse (Latin hippocampus, from Greek ἱππόκαμπος (latinised: ìppókampos) from ἵππος (ippos), 'horse' + κάμπος (kámpos), 'sea monster'). The German anatomist Johann Georg Duvernoy (1729), the first to illustrate the structure, also wavered between "seahorse" and "silkworm". "Ram's horn" was proposed by the Danish anatomist Jacob Winsløw in 1732; and a decade later his fellow Parisian, the surgeon de Garengeot, used cornu Ammonis – horn of Amun, after the ancient Egyptian god who was often represented as having a ram's head. Ammon is the Greek name for Amun.

The head region of the hippocampus is enlarged, and presents two or three rounded elevations or foot-like digitations, and hence it was named the pes hippocampi (pes meaning foot). Later this part was described as pes hippocampi major, with an adjacent bulge in the occipital horn of the lateral ventricle, described as pes hippocampi minor later renamed as the calcar avis. In 1786 Félix Vicq-d'Azyr published an authoritative description naming just the hippocampus but the term remained largely unused with no description of any function proposed until in the middle of the 20th century it was associated with memory.

Mayer mistakenly used the term hippopotamus in 1779, and was followed by some other authors until Karl Friedrich Burdach resolved this error in 1829. In 1861 the hippocampus minor became the center of a dispute over human evolution between Thomas Henry Huxley and Richard Owen, satirized as the Great Hippocampus Question. The term hippocampus minor fell from use in anatomy textbooks and was officially removed in the Nomina Anatomica of 1895. The modern term for the structure is just the hippocampus, with the term cornu Ammonis (that is, 'Ammon's horn') surviving in the names of the hippocampal subfields CA1–CA4.

The hippocampus is one of the structures of the limbic lobe, first described by Broca in 1878, as the cortical areas that line the deep edge of the cerebral cortex. The limbic lobe is the main component of the limbic system. The cingulate gyrus, and the parahippocampal gyrus are the two main parts of the described lobe, which had been largely associated with olfaction. Many studies later culminating in work by Papez, and MacLean, the involvement of other interacting brain regions associated with emotion was recognized. The hippocampus is anatomically connected to parts of the brain that are involved with emotional behavior, including the septal area, the hypothalamic mammillary bodies, and the anterior nuclear complex in the thalamus. MacLean proposed that the associated structures of the limbic lobe be included in what he termed as the limbic system.