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.
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   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.
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   3. (Astron.) The constellation Sextans.
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   Box sextant, a small sextant inclosed in a cylindrical case
      to make it more portable.
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