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]

WordNet (r) 3.0 (2006):

octant
    n 1: a measuring instrument for measuring angles to a celestial
         body; similar to a sextant but with 45 degree calibration