The Collaborative International Dictionary of English v.0.48:

Sector \Sec"tor\, n. [L., properly, a cutter, fr. secare,
   sectum, to cut: cf. F. secteur. See Section.]
   1. (Geom.) A part of a circle comprehended between two radii
      and the included arc.
      [1913 Webster]

   2. A mathematical instrument, consisting of two rulers
      connected at one end by a joint, each arm marked with
      several scales, as of equal parts, chords, sines,
      tangents, etc., one scale of each kind on each arm, and
      all on lines radiating from the common center of motion.
      The sector is used for plotting, etc., to any scale.
      [1913 Webster]

   3. An astronomical instrument, the limb of which embraces a
      small portion only of a circle, used for measuring
      differences of declination too great for the compass of a
      micrometer. When it is used for measuring zenith distances
      of stars, it is called a zenith sector.
      [1913 Webster]

   Dip sector, an instrument used for measuring the dip of the
      horizon.

   Sector of a sphere, or Spherical sector, the solid
      generated by the revolution of the sector of a circle
      about one of its radii, or, more rarely, about any
      straight line drawn in the plane of the sector through its
      vertex.
      [1913 Webster]