Longman Dictionary of Contemporary English
n. (context football English) An area of a soccer pitch extending 18 yards out of the goal, inside which a penalty is given to the offensive team if a foul is made by the defensive team.
The penalty area or 18-yard box (also known less formally as "the penalty box" or simply "the box") is an area of an association football pitch. It is rectangular and extends 16.5m (18 yd) to each side of the goal and 16.5m (18 yd) in front of it. Within the penalty area is the penalty spot (or penalty mark), which is or 12 yards from the goal line, directly in-line with the centre of the goal. A penalty arc adjoins the penalty area, and encloses the area within 9.15m (10 yd) from the penalty spot; it does not form part of the penalty area and is only of relevance during the taking of a penalty kick.
Previously, penalty areas extended the width of the field, but were reduced to their current dimensions in 1901.