Definition of HIPPOCAMPUS. : a curved elongated ridge that extends over the floor of the descending horn of each lateral ventricle of the brain, that consists of gray matter covered on the ventricular surface with white matter, and that is involved in forming, storing, and processing memory.— hip·po·cam·pal \-pəl\ adjective. ADVERTISEMENT. a curved elongated ridge that extends over the floor of the descending horn of each lateral ventricle of the brain, that consists of gray matter covered on the ventricular surface with white matter, and that is involved in forming, storing, and processing memory. — hip·po·cam·pal \-pəl\ adjective.