octant

noun

1. an eighth of a circle; 45° angle or arc
2. an instrument like the sextant, for measuring angles
3. Astron. the position of one celestial body when it is 45° distant from another
4. Math. any of the eight parts into which a space is divided by three planes intersecting at a single point and at right angles to one another

Etymology: LL octans, eighth part < L octo, eight