1560s, in geometry, "a section of a circle between two radii," from Late Latin sector "section of a circle," in classical Latin "a cutter, one who cuts," from sectus, past participle of secare "to cut" (from PIE root *sek- "to cut"). Sector translated Greek tomeus in Latin editions of Archimedes.
By 1715 of any figure having the shape of a sector; the meaning "area, division" (without regard to shape) is by 1920, perhaps generalized from a World War I military sense (1916) of "part of a front," based on a circle centered on a headquarters. The meaning "a branch of an economy" is by 1937. As a verb from 1884, "divide into sectors." Related: Sectored; sectoral; sectorial.
"a movement in a circle," 1620s, from French circuite, from Medieval Latin circuitus (see circuitous) on model of gratuite, etc.
late 14c., "a quarter of a day, six hours," from Old French quadrant, cadran, name of a Roman coin, also "a sundial," from Latin quadrantem (nominative quadrans) "a fourth part, a quarter," also the name of a coin worth a quarter of an as; noun use of the present participle of quadrare "to make square; put in order, arrange, complete; run parallel, be exact," figuratively "to fit, suit, be proper," related to quadrus "a square," quattuor "four" (from PIE root *kwetwer- "four").
From 1570s as "the quarter of a circle, the arc of a circle containing 90 degrees." The ancient surveying instrument for measuring altitudes is so called from c. 1400, because it forms a quarter circle. Related: Quadrantal.
1847, "to drive into a corral," from corral (n.). From 1848 as "to form a circle with wagons." Meaning "to lay hold of, collar, capture, make a prisoner of" is U.S. slang from 1860. Related: Corralled.
1530s, "general course of instruction," from Modern Latin encyclopaedia (c. 1500), thought to be a false reading by Latin authors of Greek enkyklios paideia taken as "general education," but literally "training in a circle," i.e. the "circle" of arts and sciences, the essentials of a liberal education; from enkyklios "circular," also "general" (from en "in;" see in + kyklos "circle;" from PIE root *kwel- (1) "revolve, move round") + paideia "education, child-rearing," from pais (genitive paidos) "child" (see pedo-).
Modern sense of "reference work arranged alphabetically" is from 1640s, often applied specifically to the French Encyclopédie ou Dictionnaire raisonné des Sciences, des Arts, et des Métiers (1751-65). Related: Encyclopedist.