Etymology
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sect (n.)
mid-14c., "distinctive system of beliefs or observances; party or school within a religion," from Old French secte, sete "sect, religious community," or directly from Late Latin secta "religious group, sect in philosophy or religion," from Latin secta "manner, mode, following, school of thought," literally "a way, road, beaten path," from fem. of sectus, variant past participle of sequi "follow," from PIE root *sekw- (1) "to follow." Confused in this sense with Latin secta, fem. past participle of secare "to cut" (from PIE root *sek- "to cut"). Meaning "separately organized religious body" is recorded from 1570s.
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sectarian (adj.)
1640s, originally applied by Presbyterians to Independents, from Medieval Latin sectarius, from secta (see sect).
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sectarianism (n.)
1670s, "disposition to petty sects in opposition to things established" [Johnson]; see sectarian + -ism.
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sectary (n.)
"member or adherent of a sect," 1550s, from French sectaire or directly from Medieval Latin sectarius, from secta (see sect).
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section (n.)
late 14c., "intersection of two straight lines; division of a scale;" from Old French section or directly from Latin sectionem (nominative sectio) "a cutting, cutting off, division," noun of action from past participle stem of secare "to cut," from PIE root *sek- "to cut." From 1550s as "act of cutting or dividing." Meaning "subdivision of a written work, statute, etc." is from 1570s. Meaning "a part cut off from the rest" is from early 15c.
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section (v.)
"divide into sections," 1819, from section (n.). Related: Sectioned; sectioning.
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sectional (adj.)
1806; see section (n.) + -al (1). Noun meaning "piece of furniture composed of sections which can be used separately" is attested by 1961, from sectional seat, sectional sofa, etc. (1949).
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sectionalism (n.)
1836, American English, from sectional + -ism. In frequent use from 1856.
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sector (n.)
1560s, "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"). Translated Greek tomeus in Latin 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. As a verb from 1884. Related: Sectoral; sectorial.
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secular (adj.)
Origin and meaning of secular

c. 1300, "living in the world, not belonging to a religious order," also "belonging to the state," from Old French seculer (Modern French séculier), from Late Latin saecularis "worldly, secular, pertaining to a generation or age," from Latin saecularis "of an age, occurring once in an age," from saeculum "age, span of time, lifetime, generation, breed."

This is from Proto-Italic *sai-tlo-, which, according to Watkins, is PIE instrumental element *-tlo- + *sai- "to bind, tie" (see sinew), extended metaphorically to successive human generations as links in the chain of life. De Vaan lists as a cognate Welsh hoedl "lifespan, age." An older theory connected it to words for "seed," from PIE root *se- "to sow" (see sow (v.), and compare Gothic mana-seþs "mankind, world," literally "seed of men").

Used in ecclesiastical writing like Greek aiōn "of this world" (see cosmos). It is source of French siècle. Ancient Roman ludi saeculares was a three-day, day-and-night celebration coming once in an "age" (120 years). In English, in reference to humanism and the exclusion of belief in God from matters of ethics and morality, from 1850s.

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