Etymology
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pruritus (n.)

"affection of the skin characterized by simple itching without visible eruption," 1650s, from Latin pruritus, past participle of prurire "to itch" (see prurient). The word was earlier in English via Old French in form prurite (early 15c.). Related: Pruritic.

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apert (adj.)
"open, evident, undisguised," early 14c., from Old French apert "obvious, evident, visible, plain to see," and directly from Latin apertus "open, uncovered, unclosed," past participle of aperire "to open, uncover" (see overt). Related: Apertly.
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discernible (adj.)

also discernable, "perceptible, visible, observable," 1560s, from French discernable, from discerner "distinguish (between), separate" (see discern). Form with -a- was more common at first; spelling changed to -i- 17c. to conform to Late Latin discernibilis. Related: Discernibly.

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catoptric (adj.)
"pertaining to mirrors or a mirror," 1774, from Latinized form of Greek katoptrikos, from katoptron "mirror," from kata "against" (see cata-) + stem of optos "seen, visible" (from PIE root *okw- "to see") + instrumental suffix -tron. Related: Catoptrics; catoptrical.
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boundary (n.)
"that which indicates the limits of anything," 1620s, from bound (n.1) + -ary. Strictly, a visible mark indicating a dividing line, a bound being the limit or furthest point of extension of any one thing.
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tarbrush (n.)
1711, from tar (n.1) + brush (n.1). To have a touch of the tarbrush "have a dash of African ancestry visible in the skin tone" (1796) was "a term of contempt from the West Indies" [Century Dictionary].
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tarn (n.)
late 14c., mid-13c. in Anglo-Latin, from Old Norse tjörn "small mountain lake without visible tributaries," from Proto-Germanic *terno, perhaps originally "water hole" [Barnhart]. A dialectal word popularized by the Lake poets.
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betoken (v.)

late Old English betacnian "to denote, to mean, signify; be a visible sign or emblem of," from be- + Old English tacnian "to signify," from tacn "sign" (see token) or from Proto-Germanic *taiknōjanan. From c. 1200 as "to augur, presage, portend," also "be or give evidence of." Related: Betokened; betokening.

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bold-face (n.)
also boldface, in typography, "having a thick or fat face," 1845, from bold (adj.) + face (n.). In reference to types, bold (adj.) is attested from 1790, perhaps from its secondary sense "easily visible, striking to the eye."
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spectrum (n.)
1610s, "apparition, specter," from Latin spectrum (plural spectra) "an appearance, image, apparition, specter," from specere "to look at, view" (from PIE root *spek- "to observe"). Meaning "visible band showing the successive colors, formed from a beam of light passed through a prism" first recorded 1670s. Figurative sense of "entire range (of something)" is from 1936.
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