\Quad"rant\, n. [L. quadrans, -antis, a fourth part, a
fourth of a whole, fr. quattuor four: cf. F. quadrant,
cadran. See {Four}, and cf. {Cadrans}.]
1. The fourth part; the quarter. [Obs.] --Sir T. Browne.
2. (Geom.) The quarter of a circle, or of the circumference
   of a circle, an arc of 90[deg], or one subtending a right
   angle at the center.
3. (Anal. (Geom.) One of the four parts into which a plane is
   divided by the co["o]rdinate 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.
4. 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[deg], 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.
{Gunner's quadrant}, an instrument consisting of a graduated
   limb, with a plumb line or spirit level, and an arm by
   which it is applied to a cannon or mortar in adjusting it
   to the elevation required for attaining the desired range.
{Gunter's quadrant}. See {Gunter's quadrant}, in the
   Vocabulary.
{Hadley's quadrant}, a hand instrument used chiefly at sea to
   measure the altitude of the sun or other celestial body in
   ascertaining the vessel's position. It consists of a frame
   in the form of an octant having a graduated scale upon its
   arc, and an index arm, or alidade pivoted at its apex.
   Mirrors, called the index glass and the horizon glass, are
   fixed one upon the index arm and the other upon one side
   of the frame, respectively. When the instrument is held
   upright, the index arm may be swung so that the index
   glass will reflect an image of the sun upon the horizon
   glass, and when the reflected image of the sun coincides,
   to the observer's eye, with the horizon as seen directly
   through an opening at the side of the horizon glass, the
   index shows the sun's altitude upon the scale; -- more
   properly, but less commonly, called an octant.
{Quadrant of altitude}, an appendage of the artificial globe,
   consisting of a slip of brass of the length of a quadrant
   of one of the great circles of the globe, and graduated.
   It may be fitted to the meridian, and being movable round
   to all points of the horizon, serves as a scale in
   measuring altitudes, azimuths, etc.