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.]
The American Heritage® Dictionary of the English Language, Fifth Edition copyright ©2018 by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
The American Heritage Dictionary Blog
Check out our blog, updated regularly, for new words and revised definitions, interesting images from the 5th edition, discussions of usage, and more.