Wiktionary
n. In photography, the distance in front of and behind the subject that appears to be in focus.
Wikipedia
In optics, particularly as it relates to film and photography, depth of field (DOF), also called focus range or effective focus range, is the distance between the nearest and farthest objects in a scene that appear acceptably sharp in an image. Although a lens can precisely focus at only one distance at a time, the decrease in sharpness is gradual on each side of the focused distance, so that within the DOF, the unsharpness is imperceptible under normal viewing conditions.
In some cases, it may be desirable to have the entire image sharp, and a large DOF is appropriate. In other cases, a small DOF may be more effective, emphasizing the subject while de-emphasizing the foreground and background. In cinematography, a large DOF is often called deep focus, and a small DOF is often called shallow focus.
Usage examples of "depth of field".
The pilot felt like God, with a total view of the heavens and fantastic depth of field.
They have great depth of field, so by setting them for 30 feet they're effective from 20 feet to infinity.
The darker figure stopped, looking up at one of the first-floor windows, and even at this distance and with no depth of field I could see his awkwardness as he waved his hand.