a space through which light passes in an optical or photographic instrument, especially the variable opening by which light enters a camera. this is very important but also quite hard and confusing to learn as the more light you want in the smaller the aperture number is. Aperture is sort of like shutter speed.
An aperture is a hole or an opening through which light travels. More specifically, the aperture and focal length of an optical system determine the cone angle of a bundle of rays that come to a focus in the image plane.