Aperture is the opening through which light is admitted into the camera and in photography it refers to the diameter of the aperture.
Some links on aperture:
Measurement of aperture is in f-number = the ratio of the focal length to the aperture diameter.
Calculation of f-number: where f is the focal length, and D is the diameter of the entrance pupil
- f/1 (big)
- f/128 (small)