n.1 an arc of a circle equal to one eighth of the circumference.
2 such an arc with two radii, forming an area equal to one eighth of the circle.
3 each of eight parts into which three planes intersecting (esp. at right angles) at a point divide the space or the solid body round it.
4 Astron. a point in a body's apparent course 45° distant from a given point, esp. a point at which the moon is 45° from conjunction or opposition with the sun.
5 an instrument in the form of a graduated eighth of a circle, used in astronomy and navigation.
Etymology: L octans octant- half-quadrant f. octo eight