late 14c., "membrane enclosing the eyeball;" c. 1400, "innermost coating of the back of the eyeball;" from Medieval Latin retina "the retina," probably from Vulgar Latin (tunica) *retina, literally "net-like tunic," on resemblance to the network of blood vessels at the back of the eye, and ultimately from Latin rete "net" (see rete).
The Vulgar Latin phrase might be Gerard of Cremona's 12c. translation of Arabic (tabaqa) shabakiyyah "netlike (layer)," itself probably a translation of Greek amphiblēstroeidēs (khiton).
word-forming element in medicine denoting "diseases characterized by inflammation" (of the specified part), Modern Latin, from Greek -itis, feminine of adjectival suffix -ites "pertaining to." Feminine because it was used with an implied nosos "disease," a feminine noun; especially in arthritis (nosos) "(disease) of the joints." Arthritis (16c.) was one of the earliest appearances of the suffix in English and from it the suffix was abstracted in other uses.
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Harper, D. (n.d.). Etymology of retinitis. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/retinitis