a. A circular arc of 90°; one fourth of the circumference of a circle.
b. The plane area bounded by such an arc and two perpendicular radii.
c. Any of the four areas into which a plane is divided by the reference axes in a Cartesian coordinate system, designated first, second, third, and fourth, counting counterclockwise from the area in which both coordinates are positive.
2. A machine part or other mechanical device that is shaped like a quarter circle.
3. An early instrument for measuring altitude of celestial bodies, consisting of a 90° graduated arc with a movable radius for measuring angles.
[Middle English, quarter of a day, from Latin quadrāns, quadrant-, a fourth part; see kwetwer- in the Appendix of Indo-European roots.]
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