NOUN (2)

1. a unit of angular distance equal to 60 degrees;

2. a measuring instrument for measuring the angular distance between celestial objects; resembles an octant;

The Collaborative International Dictionary of English v.0.48:

Sextant \Sex"tant\, n. [L. sextans, -antis, the sixth part of an
   as, fr. sextus sixth, sex six. See Six.]
   1. (Math.) The sixth part of a circle.
      [1913 Webster]

   2. An instrument for measuring angular distances between
      objects, -- used esp. at sea, for ascertaining the
      latitude and longitude. It is constructed on the same
      optical principle as Hadley's quadrant, but usually of
      metal, with a nicer graduation, telescopic sight, and its
      arc the sixth, and sometimes the third, part of a circle.
      See Quadrant.
      [1913 Webster]

   3. (Astron.) The constellation Sextans.
      [1913 Webster]

   Box sextant, a small sextant inclosed in a cylindrical case
      to make it more portable.
      [1913 Webster]

WordNet (r) 3.0 (2006):

sextant
    n 1: a unit of angular distance equal to 60 degrees
    2: a measuring instrument for measuring the angular distance
       between celestial objects; resembles an octant

Moby Thesaurus II by Grady Ward, 1.0:

42 Moby Thesaurus words for "sextant":
   astrolabe, astronavigation, azimuth circle, azimuth compass, bevel,
   bevel protractor, bevel square, celestial navigation, chronometer,
   clinometer, coastwise navigation, consolan, crescent,
   dead reckoning, fix, goniometer, graphometer, half circle,
   hemicycle, line of position, loran, pantometer, pilotage,
   plane sailing, point-to-point navigation, protractor, quadrant,
   radar, radio beacon, radio navigation, radiogoniometer, sector,
   semicircle, shoran, sofar, sonar, tables, theodolite, transit,
   transit circle, transit instrument, transit theodolite