1. One of the eight regions into which three- dimensional euclidean space is divided by the coordinate planes of a cartesian coordinate system.

2. A unit of plane angle equal to 45° or π/8 radians.

A double-reflecting instrument used primarily for measuring altitudes of celestial bodies from a moving ship or airplane; has a range of 90°, and while a sextant has the capability up to 120°, the common practice applies the name sextant to both types of instruments.