Definition of octant in US English:



  • 1An arc of a circle equal to one eighth of its circumference, or the area enclosed by such an arc with two radii of the circle.

    • ‘The arc was extended from an octant to a sextant and a stout handle was added at the back of the instrument.’
    • ‘Included in those downloads are the last two octants needed to complete the color, 360-degree ‘mission success' panorama.’
    1. 1.1 Each of eight parts into which a space or solid body is divided by three planes which intersect (especially at right angles) at a single point.
      • ‘The model put forward for the action of BDL suggests that auxin is involved in determining hypophyseal cell fate at the octant stage.’
      • ‘At the octant (eight-cell) stage the embryo proper and suspensor are clearly distinct.’
    2. 1.2 An obsolete instrument in the form of a graduated eighth of a circle, used in astronomy and navigation.
      • ‘Clark used celestial observation to determine latitude, triangulating position with an octant.’
      • ‘Instead of compasses, octants, and two-pole chains, modern digital GPS units determine position and direction.’


Late 17th century: from Latin octans, octant- ‘half-quadrant’, from octo ‘eight’.