Aperture refers to the opening in a camera lens through which light passes. The f-number, also known as the f-stop, is the ratio of the lens's focal length to the diameter of the aperture. A smaller f-number indicates a larger aperture and more light entering the camera.
Numerical aparture is the light collecting capability of the optical fiber.
According to lippincott Williams, and Wilkins it is a part of subarachnoid cisterns. It recevies CSF from the 4th ventricle via the lateral aparture of Luschka.