The size of the opening in the lens whenever a picture is taken.
How it works
Pressing the shutter button of the camera will open up a hole that enables the camera image sensor to capture a view of the scene you want to take. The aperture value set determines the size of the hole. The larger the hole, the more light that gets in and vice versa.
How is it measured
Aperture is measured in f-stops (E.g. f/2.2, f/2.4, f/2.8). A bigger aperture value does not equate to a bigger opening, and instead, works the other way around. For example, f/2.4 indicates a much larger aperture than f/22.
Applications
One of the most popular applications of aperture values is the depth of field that your images will have, with part of the image being in focus, and the rest out of focus. A larger aperture will result in a shallower depth of field (a tigh tly focused area, with less of the image in focus) while a smaller aperture will provide a larger depth of field (with more of the image being in focus).
