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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optical (adj.)

1560s, "relating to or connected with the science of optics; pertaining to vision," from optic + -al (1). Of abstract art, from 1964. In astronomy, in reference to double stars that appear so only because they lie in the same line of sight from earth, by 1868. Optical illusion is attested by 1757. Related: Optically.

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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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optician (n.)

1680s, "person skilled in the science of optics, a sense now rare or obsolete; see optic + -ian. In the meaning "one who makes or sells optical instruments" it is attested from 1737, after French opticien "maker or seller of optical instruments," on the model of physician, etc.

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eye-piece (n.)

also eyepiece, "the lens or combination of lenses to which the eye is applied in an optical instrument," 1738, from eye (n.) + piece (n.1).

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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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