Octant n. : The eighth part of a circle; an arc of 45 degrees.

Octant n. : 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.

Octant n. : 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.

Octant n. : One of the eight parts into which a space is divided by three coordinate planes.

Example:
How do you find surface in 1st Octant?

Taxonomy
octant, instrument, mirrors, telescope, index arm, horizon mirror, index mirror, scale, sextant, shades, instrument makers, measuring instrument, arc, horizon, London, mariner's quadrant, vernier, line of sight.

Defination from the web:
Octant. This instrument, labelled Crichton - London, Sold by J Berry, Aberdeen, appears to have an ebony frame with ivory scale, vernier and signature plate. ... The name octant derives from the Latin octans meaning eighth part of a circle, because the instrument's arc is one eighth of a circle. ...