Etymology
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density (n.)

c. 1600, "quality of being very close or compact," from French densité (16c.), from Old French dempsité (13c.), from Latin densitas "thickness," from densus "thick, dense" (see dense). In physics, "the mass of matter per unit of bulk," 1660s.

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spissitude (n.)

"density, thickness, compactness," mid-15c., from Latin spissitudo "thickness, density," from spissus "thick, dense, compact, close" (source of Italian spesso, Spanish espeso, Old French espes, French épais). Related: Spissated.

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dord (n.)

a ghost word printed in the 1934 "Webster's New International Dictionary" and defined as a noun used by physicists and chemists, meaning "density." In sorting out and separating abbreviations from words in preparing the dictionary's second edition, a card marked "D or d" meaning "density" somehow migrated from the "abbreviations" stack to the "words" stack. The "D or d" entry ended up being typeset as a word, dord, and defined as a synonym for density. The mistake was discovered in 1939.

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thickness (n.)

Old English þicness "density, viscosity, hardness; depth; anything thick or heavy; darkness; thicket;" see thick + -ness.

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densimeter (n.)

"apparatus for ascertaining the comparative density of a solid or liquid," 1848; see dense + -meter.

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pycnometer (n.)

"instrument for determining the relative density of solid bodies," by 1858; see pycno- + -meter.

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tesla (n.)

"unit of magnetic flux density," 1960, from Nikola Tesla (1856-1943), Croatian-born U.S. engineer. Tesla coil is attested from 1896.

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rarefy (v.)

late 14c., rarefien, "make thin, reduce the density of," from Old French rarefier (14c.) and directly from Medieval Latin rarificare, from Latin rarefacere "make thin, make rare," from rarus "rare, thin" (see rare (adj.1)) + facere "to make" (from PIE root *dhe- "to set, put"). Intransitive sense of "become less dense" is from 1650s. Related: Rarefied; rarefiable.

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consistence (n.)

1590s, "state of standing still; firmness," from French consistence (Modern French consistance) "a standing fast," from Medieval Latin consistentia, literally "a standing together," from Latin consistentem (nominative consistens), present participle of consistere "to stand firm, take a standing position, stop, halt," from assimilated form of com "with, together" (see con-) + sistere "to place," causative of stare "to stand, be standing" (from PIE root *sta- "to stand, make or be firm"). Meaning "coherence, solidity, state or degree of density" is recorded from 1620s.

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frontier (n.)

c. 1400, frowntere, "front line of an army;" early 15c., fronture, "borderland, part of a country which faces another," from Old French frontiere "boundary-line of a country," also "frontier fortress; front rank of an army" (13c.), noun use of adjective frontier "facing, neighboring," from front "brow" (see front (n.)). In reference to North America, "part of the country which is at the edge of its settled regions" from 1670s. Later it was given a specific sense:

What is the frontier? ... In the census reports it is treated as the margin of that settlement which has a density of two or more to the square mile. [F.J. Turner, "The Frontier in American History," 1920]
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