Etymology
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displacement (n.)

1610s, "removal from office;" see displace + -ment. As "quantity of a liquid displaced by a solid body put into it," 1809. Physics sense "amount by which anything is displaced" is from 1837.

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unit (n.)

1560s, "single number regarded as an undivided whole," alteration of unity on the basis of digit. Popularized in John Dee's English translation of Euclid, to express Greek monas (Dee says unity formerly was used in this sense). Meaning "single thing regarded as a member of a group" is attested from 1640s. Extended sense of "a quantity adopted as a standard of measure" is from 1738. Sense of "group of wards in a hospital" is attested from 1893.

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parallax (n.)

"apparent displacement of an object observed, due to an actual displacement of the observer," 1570s, from French parallaxe (mid-16c.), from Greek parallaxis "change, alteration, inclination of two lines meeting at an angle," from parallassein "to alter, make things alternate," from para- (see para- (1)) + allassein "to change," from allos "other" (from PIE root *al- "beyond"). Related: Parallactic.

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labile (adj.)

mid-15c., "prone to lapse," from Latin labilis, from labi "to slip" (see lapse (n.)). Hence, in chemistry, "prone to undergo displacement" (c. 1600).

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photon (n.)

"unit of electromagnetic radiation," 1926, from photo- "light" + -on "unit." Related: Photonic.

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ectopic (adj.)

1864 in reference to pregnancy, from ectopia "morbid displacement of parts" (1847), coined in Modern Latin from Greek ektopos "away from a place, distant; foreign, strange," from ek- "out" (see ex-) + topos "place" (see topos).

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unitary (adj.)

1847, "characterized by unity or uniformity;" 1865, "of or relating to a unit;" see unit + -ary.

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monad (n.)

1610s, "unity, arithmetical unit," 1610s, from Late Latin monas (genitive monadis), from Greek monas "unit," from monos "alone" (from PIE root *men- (4) "small, isolated"). In Leibnitz's philosophy, "an ultimate unit of being, a unit of the universal substance" (1748); he apparently adopted the word from Giordano Bruno's 16c. metaphysics, where it referred to a hypothetical primary indivisible substance at once material and spiritual. Related: Monadic; monadism.

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B.T.U. 

1889 as an abbreviation of British Thermal Unit (1862), a commercial unit of electrical energy (the amount of heat required to raise the temperature of one pound of water by one degree Fahrenheit); the French Thermal Unit is the amount of heat required to raise 1 kilogram of water 1 degree centigrade. Also from 1889 as an abbreviation of Board of Trade Unit, in electicity "1,000 watt hours."

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lira (n.)

Italian monetary unit, 1610s, from Italian lira, literally "pound," from Latin libra "pound (unit of weight);" see Libra, and compare livre. There also was a Turkish lira.

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