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From 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]