Depth of field preview
A function available on some digital single-lens reflex cameras that displays a picture in the viewfinder that corresponds to the aperture that will be used to expose the photograph. This function is typically implemented as a button. This provides a visual sense of the amount of depth of field that is present, as well as which sections of the final picture will be crisp and which parts will be blurry. This is important because the viewfinder typically only displays the image in the manner in which it would appear if the aperture was opened up to its widest possible setting.
Do you have a better definition? Share it in the comments.
