Geometry. a plane figure bounded by two radii and the included arc of a circle.
a mathematical instrument consisting of two flat rulers hinged together at one end and bearing various scales.
Machinery. a device used in connection with an index plate, consisting of two arms rotating about the center of the plate and set to indicate the angle through which the work is indexed.
Military. a designated defense area, usually in a combat zone, within which a particular military unit operates and for which it is responsible.
Astronomy. an instrument shaped like a sector of a circle, having a variable central angle and sights along the two straight sides, for measuring the angular distance between two celestial bodies.
a distinct part, especially of society or of a nation's economy: the housing sector; the educational sector.
a section or zone, as of a city.
Computers. a portion of a larger block of storage, as 1/128 of a track or disk.
verb (used with object)
to divide into sectors.

1560–70; < Late Latin: sector, Latin: cutter, equivalent to of sec(āre) to cut + -tor -tor

sectoral, adjective
subsector, noun Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2014.
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World English Dictionary
sector (ˈsɛktə)
1.  a part or subdivision, esp of a society or an economy: the private sector
2.  geometry either portion of a circle included between two radii and an arc. Area: ½r²θ, where r is the radius and θ is the central angle subtended by the arc (in radians)
3.  a measuring instrument consisting of two graduated arms hinged at one end
4.  a part or subdivision of an area of military operations
5.  computing the smallest addressable portion of the track on a magnetic tape, disk, or drum store
[C16: from Late Latin: sector, from Latin: a cutter, from secāre to cut]

Collins English Dictionary - Complete & Unabridged 10th Edition
2009 © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
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Word Origin & History

1570, "section of a circle between two radii," from L.L. sector "section of a circle," from L. sector "a cutter," from sectus, pp. of secare "to cut" (see section). Translated Gk. tomeus in L. editions of Archimedes. Meaning "area, division" appeared 1920, generalized from
military sense (1916) of "part of a front," based on a circle centered on a headquarters.
Online Etymology Dictionary, © 2010 Douglas Harper
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American Heritage
Science Dictionary
sector   (sěk'tər)  Pronunciation Key 
The part of a circle bounded by two radii and the arc between them.
The American Heritage® Science Dictionary
Copyright © 2002. Published by Houghton Mifflin. All rights reserved.
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