Aperture
The size of the aperture is described as an "f-stop."  This is a number that describes the size of the hole in relation to the focal length of the lens.  A low number (such as f1.4 or f2.8) means the aperture is wide open, and lets in a lot of light.  A high number (such as f8 or f11) means that the aperture is small, and does not let in as much light.