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The Collaborative International Dictionary of English v.0.48 (gcide)

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]