- a quarter of a circle; an arc of 90°.
- the area included between such an arc and two radii drawn one to each extremity.
- something shaped like a quarter of a circle, as a part of a machine.
- Geometry, Astronomy. one of the four parts into which a plane, as the face of a heavenly body, is divided by two perpendicular lines, numbered counterclockwise from upper right: the first quadrant of the moon.
- an instrument, usually containing a graduated arc of 90°, used in astronomy, navigation, etc., for measuring altitudes.
- Astrology. one of the four quarters of the horoscope: determined by the ascendant, nadir, descendant, and midheaven and numbered counterclockwise from the ascendant.
- Furniture. one of two metal sliding pieces, of quadrant form, used to support the fall front of a desk from above.