Etymology
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normal (adj.)

c. 1500, "typical, common;" 1640s, in geometry, "standing at a right angle, perpendicular," from Late Latin normalis "in conformity with rule, normal," in classical Latin "made according to a carpenter's square," from norma "rule, pattern," literally "carpenter's square," a word of unknown origin (see norm). Meaning "conforming to common standards or established order or usage, regular, usual" is attested from 1828 but probably is older than the record [Barnhart].

Meaning "heterosexual" is by 1914. As a noun meaning "usual state or condition," from 1890 (in geometry as "a perpendicular" from 1727). Sense of "a normal person or thing" is attested by 1894. Normal school "training college for teachers" (1835) is a translation of French école normale (1794), a creation of the French Republic; the notion is of "serving to set a standard." The U.S. city of Normal, Illinois, was named 1857 for the normal school established there.

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segment (n.)
1560s, from Latin segmentum "a strip or piece cut off, a cutting, strips of colored cloth," from secare "to cut" (from PIE root *sek- "to cut"), with euphonious alteration of -c- to -g- before -m-. Latin segmentum was used in Medieval Latin as a geometry term, translating Greek tmema, and the word was first picked up in English in this sense. Meaning "segmental portion of anything circular" is from 1640s; general sense of "a division, section" is from 1762.
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trivium (n.)

by 1751, from Medieval Latin trivium (9c.) "grammar, rhetoric, and logic," the first three of the seven liberal arts, considered initiatory and foundational to the other four (arithmetic, geometry, astronomy, and music). From Latin trivium, in classical Latin "place where three roads meet; a frequented place; public street, highway," from tri- "three" (see three) + via "road" (see via). Compare trivia and also see quadrivium.

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parabola (n.)

"a curve commonly defined as the intersection of a cone with a plane parallel with its side," 1570s, from Modern Latin parabola, from Greek parabole "a comparison, parable," literally "a throwing beside," hence "a juxtaposition" (see parable), so called by Apollonius of Perga c. 210 B.C.E. because it is produced by "application" of a given area to a given straight line. It had a different sense in Pythagorean geometry. Related: Parabolic.

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chord (n.2)

"structure in animals resembling a string," 1540s, alteration of cord (n.), by influence of Greek khorde "gut-string, string of a lyre, tripe," from PIE root *ghere- "gut, entrail."

Meaning "string of a musical instrument" is from 1660s (earlier this was cord). The geometry sense "straight line intersecting a curve" is from 1550s; figurative meaning "feeling, emotion" first attested 1784, from the notion of the heart or mind as a stringed instrument.

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congruent (adj.)

early 15c., "suitable, proper, harmoniously joined or related," from Latin congruentem (nominative congruens) "agreeing, fit, suitable," present participle of congruere "agree, correspond with," literally "to come together," from assimilated form of com "with, together" (see con-) + a lost verb *gruere, *ruere "fall, rush," which de Vaan traces to a PIE *ghr(e)uho- "to rush in." 

Geometry sense, "capable of being superposed," is attested by 1706. Related: Congruently.

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gnomon (n.)

"vertical shaft that tells time by the shadow it casts" (especially the triangular plate on a sundial), 1540s, from Latin gnomon, from Greek gnōmōn "indicator (of a sundial), carpenter's rule," also, in plural, "the teeth that mark the age of a horse or mule," literally "one that discerns or examines, interpreter, expert," from gignōskein "to come to know," from PIE root *gno- "to know." In geometry from 1560s, from a use in Greek. In early use in English sometimes folk-etymologized as knowman. Related: Gnomonic.

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tierce (n.)
old unit of liquid measure equal to one-third of a pipe (42 gallons), 1530s, from Anglo-French ters, Old French tierce (11c.). used in the sense "one-third" in various ways, from Latin tertia, fem. of tertius "a third," from PIE *tri-tyo-, from root *trei- (see three). Also used in Middle English for "a third part" (late 15c.), "the third hour of the canonical day" (ending at 9 a.m.), late 14c., and, in astronomy and geometry, "sixtieth part of a second of an arc."
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quadrivium (n.)

"arithmetic, music, geometry, astronomy" (the four branches of mathematics, according to the Pythagoreans), by 1751, from Latin quadrivium, which meant "place where four roads meet, crossroads," from quadri- "four" (from PIE root *kwetwer- "four") + via "way, road, channel, course" (see via). Compare liberal arts, and also see trivium.

The adjective quadrivial is attested from mid-15c. in English with the sense of "belonging to the quadrivium," late 15c. with the sense of "having four roads, having four ways meeting in a point."

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logarithm (n.)

a mathematical function used to shorten calculation, 1610s, logarithmus, coined in Modern Latin by Scottish mathematician John Napier (1550-1617), literally "ratio-number," from Greek logos "proportion, ratio, word" (see Logos) + arithmos "number" (from PIE *erei-dhmo-, suffixed variant form of root *re- "to reason, count"). Napier invented them and published a table in 1614; the kind now chiefly in use were invented by his contemporary Henry Briggs (1561-1630), a professor of geometry at Gresham College, London.

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