The aperture is the term used to describe the size of the hole in the front of the camera that permits light to enter and fall on the film (or the electronic image sensor) that is collecting the light. The device that controls the size of the aperture is known as the iris. Hence the setting that measures the iris 'hole' size is known as the aperture.

Aperture is measured in f-stops and these range from f1.4 (fully open) through f2.0, f2.8, f4, f5., f8, f11 and f16, right up to f22 (fully closed). I.e. the larger the f-stop number the smaller the aperture size-the smaller the size of the hole permitting light into the camera. f1 .4 permits twice as much light as f2.0 and f2 .0 permits twice as much lighting as f2 .8.