Octant

n.	1.	(Geom.) The eighth part of a circle; an arc of 45 degrees.
	2.	(Astron. & Astrol.) The position or aspect of a heavenly body, as the moon or a planet, when half way between conjunction, or opposition, and quadrature, or distant from another body 45 degrees.
	3.	An instrument for measuring angles (generally called a quadrant), having an arc which measures up to 9O°, but being itself the eighth part of a circle. Cf. Sextant.
	4.	(Math. & Crystallog.) One of the eight parts into which a space is divided by three coördinate planes.

Noun

1.

octant - a measuring instrument for measuring angles to a celestial body; similar to a sextant but with 45 degree calibration

limb, measuring device, measuring instrument, measuring system