Etymology
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irradiance (n.)

1660s, from Latin irradiantem (nominative irradians), present participle of irradiare "to shine forth" (see irradiate). Related: Irradiancy (1640s).

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irradiant (adj.)

1520s, from Latin irradiantem (nominative irradians), present participle of irradiare "to shine forth" (see irradiate). Related: Irradiantly.

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cteno- 

word-forming element used in biological and zoological compounds, from Latinized form of Greek ktenidion "a little comb," diminutive of kteis "comb," from PIE root *pekt-en- "comb" (source also of Latin pecten "comb").

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peignoir (n.)

"lady's loose robe," 1835, from French peignoir, from Middle French peignouoir "loose, washable garment worn over the shoulders while combing the hair" (16c.), from peigner "to comb the hair," from Latin pectinare, from pecten (genitive pectinis) "a comb," related to pectere "to comb" (see fight (v.)). A gown put on while coming from the bath; misapplied in English to a woman's morning gown.

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pectinate (adj.)

"having teeth like a comb," 1793," from Latin pectinatus, past participle of pectinare, from pecten "a comb," from PIE *p(e)tk- "to comb" (source also of Greek pekein, pektein "to comb, shear," Lithuanian pėšti "to pluck," Old High German fehtan "to fight;" see fight (v.)). Related: Pectination; pectineal. As a verb, "to fit together in a relation" (like the teeth of two combs), 1640s.

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