Etymology
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flux (n.)

late 14c., "abnormally copious flow," from Old French flus "a flowing, a rolling; a bleeding" (Modern French flux), or directly from Latin fluxus (adj.) "flowing, loose, slack," past participle of fluere "to flow" (see fluent). Originally "excessive flow" (of blood or excrement), it also was an early name for "dysentery;" sense of "continuous succession of changes" is first recorded 1620s. The verb is early 15c., from the noun.

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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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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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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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rheumatism (n.)

1680s as a name applied to various similar diseases causing inflammation and pain in the joints, from Late Latin rheumatismus, from Greek rheumatismos, from rheumatizein "suffer from the flux," from rheuma "a discharge from the body" (see rheum). "The meaning of a disease of the joints is first recorded in 1688, because rheumatism was thought to be caused by an excessive flow of rheum into a joint thereby stretching ligaments" [Barnhart].

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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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