Lateral ventricle: A communicating cavity in the brain that is part of a system of four communicating cavities that are continuous with the central canal of the spinal cord. The two lateral ventricles are located in the cerebral hemispheres, one in each hemisphere. Each consists of a triangular central body and four horns. The third and fourth ventricles are located in the center of the brain. The lateral ventricles communicate with the third ventricle through an opening called the interventricular foramen. Both lateral ventricles are filled with cerebrospinal fluid.