Source : Webster's Revised Unabridged Dictionary (1913)

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.

   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[deg], 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 co["o]rdinate planes.

Source : WordNet®

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