1. An arrow for a crossbow; -- so named because it commonly had a square head. [Obs.] To shoot with arrows and quarrel. Sir J. Mandeville. Two arblasts, . . . with windlaces and quarrels. Sir W. Scott. 2. (Arch.) Any small square or quadrangular member; as: (a) A square of glass, esp. when set diagonally. (b) A small opening in window tracery, of which the cusps, etc., make the fo...