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# Definition of: octant

(ok**′**tənt) noun **1.** An eighth part of a circle; an arc subtending an angle of 45 degrees. **2.** Astron. The position in the heavens that is one eighth of a circle distant from conjunction or quadrature; one of the four positions of the moon midway between new or full moon and quarters. **3.** An instrument resembling a sextant but having an arc of only 45 degrees: used for measuring the angular height of the sun, moon, and other celestial bodies as an aid in navigation. **4.** Geom. One of the eight trihedral compartments of space formed by three planes with the three axes, *x, y*, and *z*, of a Cartesian coordinate system as edges. [<LL *octans* an eighth part <L *octo* eight] **—oc·tant′al** (-tan

**′**təl)

*adjective*

CARTESIAN OCTANT

*O.*Origin. Coordinates of

*A: x*= 4, =

*y*= 7,

*z*= 5; of

*B; x*= 4,

*y*= 7.