Quadrant  n. 1 quarter of a circle's circumference. 2 quarter of a circle enclosed by two radii at right angles. 3 quarter of a sphere etc. 4 any of four parts of a plane divided by two lines at right angles. 5 a graduated quarter-circular strip of metal etc. B instrument graduated (esp. Through an arc of 90°) for measuring angles. [latin quadrans -ant-]