Aperture may refer to any of the following:
1. In optics, aperture is a hole or an opening through which light travels that focuses an image. In photography, the lens aperture is usually specified as an f-number (e.g., f/2.8), which is the ratio of focal length to effective aperture diameter. A lower f-number has a larger aperture opening that allows more light to reach the image sensor.
With a small aperture (large f-numbers), you get an image with everything relatively in focus, which is good for landscape type shot. With a large aperture (small f-numbers), your subject is in focused with everything else blurred, which is good for a portrait shot.
Aperture Priority AE sometimes abbreviated as "A" or "Av" (aperture value) is a camera option for selecting an aperture opening to auto-adjust the shutter speed. Adjusting the Aperture Priority AE lets you control the depth of field.
2. In Trinitron monitors, an aperture grille is a set of vertical wires stretched vertically down the inside of the CRT (cathode ray tube). The grille masks beams from the electron guns at the back of the tube. In a standard monitor, a metal plate called a shadow mask performs this function.