Etymology
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radian (n.)
"angle subtended at the center of a circle by an arc equal in length to the radius," 1879, from radius.
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coterie (n.)

"exclusive set or circle of persons who are in the habit of meeting and socializing, a clique," 1738, from French coterie "circle of acquaintances," originally an organization of peasants holding land from a feudal lord (14c.), from cotier "tenant of a cote" (see cottage).

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esoterica (n.)
by 1807, from Latinized plural of Greek esoterikos "belonging to an inner circle, pertaining to those within" (see esoteric).
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gyre (v.)
mid-15c., "turn (something) away (from something else); rotate" (transitive), "cause to revolve;" also "go in a circle, turn round" (intransitive), from Old French girer and directly from Latin gyrare, verb derived from gyrus "circle, circular course, round, ring" (see gyre (n.)). Related: Gyred; gyring.
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clachan (n.)

"small village" (Scottish and Irish), early 15c., from Gaelic clach (plural clachan) "stone," originally perhaps "a stone circle."

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diameter (n.)

late 14c., in geometry, "chord of a circle or sphere which passes through its center; the length of a diameter," from Old French diametre, from Latin diametrus, from Greek diametros (gramme) "diagonal of a circle," from dia "across, through" (see dia-) + metron "a measure" (from PIE root *me- (2) "to measure").

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yuan (n.)
Chinese unit of currency introduced 1914, from Chinese yuan "round, round object, circle."
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unicycle (n.)
1869, American English, from Latin uni- "one" (see uni-) + -cycle, from bicycle (from Greek kyklos "circle, wheel").
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encyclical (adj.)
in reference to ecclesiastical letters meant for wide circulation (for example, a letter sent by a pope to all bishops), 1640s, from Late Latin encyclicus, from Latin encyclius, from Greek enkyklios "in a circle, circular," from en "in" (see in) + kyklos "circle" (from PIE root *kwel- (1) "revolve, move round"). As a noun, from 1837.
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cyclo- 

before a vowel, cycl-, word-forming element in technical terms meaning "circle, ring, rotation," from Latinized form of Greek kyklos "circle, wheel, ring" (from PIE root *kwel- (1) "revolve, move round"). In organic chemistry it is used in forming chemical names of cyclic compounds.

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