A square 10 kilometers by 10 kilometers on the National Grid (100 square kilometers in area) often used as the unit for mapping in National Atlases. Often referred to as '10km squares', see also Tetrad. (for completeness; a 5x5km square is apparently called a Quadrant (four to a Hectad: used in some Continental mapping schemes) and a 1x1km square is a Monad)

A hectad is an area 10 km x 10 km square.