the fourth part; the quarter.-the quarter of a circle, or of the circumference of a circle, an arc of 90¡, or one subtending a right angle at the center.-one of the four parts into which a plane is divided by the coordinate axes. the upper right-hand part is the first quadrant; the upper left-hand part the second; the lower left-hand part the third; and the lower right-hand part the fourth quadrant.-an instrument for measuring altitudes, variously constructed and mounted for different specific uses in astronomy, surveying, gunnery, etc., consisting commonly of a graduated arc of 90¡, with an index or vernier, and either plain or telescopic sights, and usually having a plumb line or spirit level for fixing the vertical or horizontal direction.