The opening through which light enters the camera. The size of the aperture can be made smaller or bigger to reduce or increase the light that can enter. In a low light situation, one would typically open up the aperture to allow more light to enter and do the reverse under bright light conditions.

The aperture is not described in terms of its actual size, but in terms of its f-number (or f-stop). The f-stop  is the ratio of the focal length of the lens to the diameter of the aperture, It is important to note that as the f-numbers increase, the size of the aperture decreases. When we say we are "stopping down the lens" that means we are increasing the f-number or decreasing the aperture area. If we are "opening up" the lens, this corresponds to decreasing the f-numbers or increasing the aperture area.

f-stops as they are typically marked on lenses are:

1   1.4   2   2.8   4   5.6   8   11   16   22    32

There is another important factor that is linked to the choice of aperture – and that is called the depth of field. A small aperture gives a good depth of field; a bigger aperture gives a shallow depth of field. There also are times when you want the depth of field to be small; for example, if objects in the background of a photograph's subject would be distracting, it is common to have them so badly out of focus that they simply become a blur, allowing the image to emphasize the subject rather than the distracting background. In such a case you would want to open up the aperture (decrease the f-number)