Dictionary:

octant

  (ŏk'tənt)

1. One eighth of a circle.
2. 1. A 45° arc.
   2. The area enclosed by two radii at a 45° angle and the intersected arc.
3. An instrument based on the principle of the sextant but employing only a 45° angle, used as an aid in navigation.
4. Astronomy. The position of a celestial body when it is separated from another by a 45° angle.
5. One of eight parts into which three-dimensional space is divided by three usually perpendicular coordinate planes.

[Latin octāns, octant-, from octō, eight.]

n. an obsolete instrument in the form of a graduated eighth of a circle, used in astronomy and navigation.

See the Introduction, Abbreviations and Pronunciation for further details.

The noun has one meaning:

Meaning #1: a measuring instrument for measuring angles to a celestial body; similar to a sextant but with 45 degree calibration

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