means A distinct part or division of something larger — whether a slice of an economy, a zone of a city, a region of a battlefield, or a wedge of a circle.

from From Latin sector, 'a cutter' or 'one who cuts,' from secare, 'to cut' — the same root that gives us section, segment, and dissect. Geometers borrowed it first for the pie-slice shape carved out between two radii of a circle, and from that clean wedge the word spread outward to carve up economies, markets, and territories into manageable pieces.