Etymology
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diagram (n.)

1610s, "an illustrative figure giving only the outlines or general scheme of the object;" 1640s in geometry, "a drawing for the purpose of demonstrating the properties of a figure;" from French diagramme, from Latin diagramma "a scale, a musical scale," from Greek diagramma "geometric figure, that which is marked out by lines," from diagraphein "mark out by lines, delineate," from dia "across, through" (see dia-) + graphein "write, mark, draw" (see -graphy). Related: Diagrammatic; diagrammatically.

The verb, "to draw or put in the form of a diagram," is by 1822, from the noun. Related: Diagrammed; diagramming.

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scheme (n.)

1550s, "figure of speech" (a sense now obsolete), from Medieval Latin schema "a shape, a figure, a form, appearance; figure of speech; posture in dancing," from Greek skhēma (genitive skhematos) "figure, appearance, the nature of a thing," which is related to skhein "to get," and ekhein "to have, hold; be in a given state or condition" (from PIE root *segh- "to hold").

By 1610s as "linear representation showing relative positions pf the parts or elements of a system" (especially in astrology). The sense "program of action" is by 1640s, also "outline, draft of a book, etc."

The meaning "plan of action devised to attain some end" is by 1718, and unfavorable overtones (selfishness, deviousness) began to creep in to the word after that time. Meaning "complex unity of coordinated component elements, a connected and orderly arrangement" is from 1736. In prosody by 1838. Color scheme is by 1890 (in Milton Bradley Co.'s "Color in the School-Room"); earlier scheme of colour (by 1877).

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crossbones (n.)

also cross-bones, "figure of two thigh-bones laid across each other in the form of an X," 1798, from cross- + bone (n.).

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nonagon (n.)

"plane figure with nine sides and nine angles," 1680s, a hybrid from Latin nonus "ninth" (from novem "nine;" see nine) + ending from pentagon, etc.

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quadrangle (n.)

late 14c., "a plane figure having four angles; a rectangle, square, etc.," from Old French quadrangle (13c.) and directly from Late Latin quadrangulum "four-sided figure," noun use of neuter of Latin adjective quadrangulus "having four corners," from Latin quattuor "four" (from PIE root *kwetwer- "four") + angulus "angle" (see angle (n.)). Meaning "four-sided court nearly surrounded by buildings" is from 1590s.

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refigure (v.)

late 14c., refiguren, "represent; represent again" (to the mind), from re- "again, back" + figure (v.) or else from Latin refigurare. Related: Refigured; refiguring; refiguration

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lune (n.)

figure formed by two arcs of circles, anything in the shape of a crescent or half-moon, 1704, from Latin luna "moon; crescent-shaped badge" (see luna).

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heart-beat (n.)

also heartbeat, 1850, "a pulsation of the heart," from heart (n.) + beat (n.). From its coinage used as a figure for "a very brief time."

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triskelion (n.)

"figure consisting of three branches radiating from a center," 1880, earlier triskelos (1857), from Greek triskeles "three-legged," from tri- "three" (see tri-) + skelos "leg" (see scoliosis).

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ellipsoid (n.)

in geometry, "a solid figure, all planes of which are ellipses or circles," 1721; see ellipse + -oid. From 1861 as an adjective (earlier adjective was ellipsoidal, 1831).

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