A part of a circle comprehended between two radii and the included arc. [1913 Webster]
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]
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.

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OXFORD DICTIONARY

sector, n.
1 a distinct part or branch of an enterprise, or of society, the economy, etc.
2 Mil. a subdivision of an area for military operations, controlled by one commander or headquarters.
3 the plane figure enclosed by two radii of a circle, ellipse, etc., and the arc between them.
4 a mathematical instrument consisting of two arms hinged at one end and marked with sines, tangents, etc., for making diagrams etc.

Derivative

sectoral adj.

Etymology

LL, techn. use of L sector cutter (as SECTION)

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THESAURUS

sector

adjunct, component, contingent, crescent, cross section, detachment, detail, district, division, dole, fraction, half circle, hemicycle, installment, item, parcel, part, particular, percentage, portion, precinct, quadrant, quarter, quota, random sample, remainder, sample, sampling, section, segment, semicircle, sextant, share, subdivision, subgroup, subspecies

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ROGET THESAURUS

sector

Part

N part, portion, dose, item, particular, aught, any, division, ward, subdivision, section, chapter, clause, count, paragraph, verse, article, passage, sector, segment, fraction, fragment, cantle, frustum, detachment, parcel, piece, lump, bit cut, cutting, chip, chunk, collop, slice, scale, lamina, small part, morsel, particle, installment, dividend, share, debris, odds and ends, oddments, detritus, excerpta, member, limb, lobe, lobule, arm, wing, scion, branch, bough, joint, link, offshoot, ramification, twig, bush, spray, sprig, runner, leaf, leaflet, stump, component part, sarmentum, compartment, department, county, fractional, fragmentary, sectional, aliquot, divided, in compartments, multifid, disconnected, partial, partly, in part, partially, piecemeal, part by part, by by installments, by snatches, by inches, by driblets, bit by bit, inch by inch, foot by foot, drop by drop, in detail, in lots.

Circularity

N circularity, roundness, rotundity, circle, circlet, ring, areola, hoop, roundlet, annulus, annulet, bracelet, armlet, ringlet, eye, loop, wheel, cycle, orb, orbit, rundle, zone, belt, cordon, band, contrate wheel, crown wheel, hub, nave, sash, girdle, cestus, cincture, baldric, fillet, fascia, wreath, garland, crown, corona, coronet, chaplet, snood, necklace, collar, noose, lasso, lassoo, ellipse, oval, ovule, ellipsoid, cycloid, epicycloid, epicycle, semicircle, quadrant, sextant, sector, sphere, round, rounded, circular, annular, orbicular, oval, ovate, elliptic, elliptical, egg-shaped, pear-shaped, cycloidal, spherical, I watched the little circles die.

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