Etymology
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geometry (n.)

early 14c., also gemetrie, gemetry, from Old French geometrie (12c., Modern French géométrie), from Latin geometria, from Greek geometria "measurement of earth or land; geometry," from combining form of "earth, land" (see Gaia) + -metria "a measuring of" (see -metry). Old English used eorðcræft "earth-craft" as a loan-translation of Latin geometria.

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geometer (n.)
"one skilled in geometry," late 15c., from Latin geometres (in Late Latin also geometra), from Greek geometres "land-measurer" (see geometry).
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geometrical (adj.)
late 14c., from Latin geometricus "of geometry" (from geometria; see geometry) + -al. Since 16c. it has been opposed to arithmetical in ratio, proportion, etc., reflecting the fact that problems of multiplication formerly were dealt with by geometry, not arithmetic. Related: Geometrically.
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gematria (n.)
1680s, from Hebrew gematriya, a transliteration of Greek geometria (see geometry). "[E]xplanation of the sense of a word by substituting for it another word, so that the numerical value of the letters constituting either word is identical" [Klein].
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*me- (2)
*mē-, Proto-Indo-European root meaning "to measure." Some words may belong instead to root *med- "to take appropriate measures."

It forms all or part of: amenorrhea; centimeter; commensurate; diameter; dimension; gematria; geometry; immense; isometric; meal (n.1) "food, time for eating;" measure; menarche; meniscus; menopause; menses; menstrual; menstruate; mensural; meter (n.1) "poetic measure;" meter (n.2) unit of length; meter (n.3) "device for measuring;" -meter; Metis; metric; metrical; metronome; -metry; Monday; month; moon; parameter; pentameter; perimeter; piecemeal; semester; symmetry; thermometer; trigonometry; trimester.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit mati "measures," matra "measure;" Avestan, Old Persian ma- "to measure;" Greek metron "measure," metra "lot, portion;" Latin metri "to measure."
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isometry (n.)
in mathematics, 1941, probably from isometric (q.v.) on the model of geometry/geometric.
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subnormal (adj.)
1875, from sub- + normal. The noun is from 1710 in geometry; 1916, of persons.
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coordinal (adj.)

also co-ordinal, 1849, in mathematics and geometry, "having (a certain number) of coordinates;" see co- + ordinal.

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decahedron (n.)

in geometry, "a solid having ten faces," 1828, from deca- "ten" + -hedron, from Greek hedra "seat, base, chair, face of a geometric solid," from PIE root *sed- (1) "to sit."  Related: Decahedral.

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reductio ad absurdum 
Latin, literally "reduction to the absurd." Absurdum is neuter of absurdus. See reduction + absurd. The tactic is useful and unobjectionable in proofs in geometry.
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