Source : Webster's Revised Unabridged Dictionary (1913)

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.

   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.

   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.

   {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.

Source : WordNet®

sector
     n 1: a plane figure bounded by two radii and the included arc of
          a circle
     2: a body of people who form part of society or economy; "the
        public sector"
     3: a particular aspect of life or activity; "he was helpless in
        an important sector of his life" [syn: {sphere}]
     4: the minimum track length that can be assigned to store
        information; unless otherwise specified a sector of data
        consists of 512 bytes
     5: a portion of a military position
     6: measuring instrument consisting of two graduated arms hinged
        at one end