Depth of field
From The Glossary of Digital Photography
The areas in front of and behind the main subject that remain acceptably sharp. A number of factors influence depth of field, including the focal length of the lens, the aperture, and the distance from the subject. A larger aperture (smaller f-stop number) will yield a smaller or shallower depth of field. A longer focal-length lens will also yield a smaller depth of field. The closer the photographer is to the subject, the narrower the depth of field will be. See also depth of focus
