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From The Collaborative International Dictionary of English v.0.48:
Octant \Oc"tant\, n. [L. octans, -antis. fr. octo eight. See Octave.] 1. (Geom.) The eighth part of a circle; an arc of 45 degrees. [1913 Webster] 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. [1913 Webster] 3. An instrument for measuring angles (generally called a quadrant), having an arc which measures up to 9O[deg], but being itself the eighth part of a circle. Cf. Sextant. [1913 Webster] 4. (Math. & Crystallog.) One of the eight parts into which a space is divided by three coordinate planes. [1913 Webster]