(n.) the thin membrane forming the wings of bats and gliding mammals
The thin layer of skin, stretched between a bat’s digits, which forms its membranous wings is called the patagium—and so too is the thicker, fur-covered membrane the forms the winds of gliding mammals like flying squirrels, anomalures, sifakas, and sugar gliders.
But that’s not all the word patagium means.
As mentioned on Twitter, in Classical Latin a patagium was a decorative trim, often in golden thread, around the fringe of a woman’s tunic. Because of the way a tunic forms an enveloping flap between a raised arm and the abdomen, the word was repurposed by nineteenth century zoologists who required a word to describe the flying membrane of flighted mammals. In that sense, it first appeared in English in 1826.
But where did the word itself originally come from? Curiously, the word is thought to be echoic in origin: several dictionaries point to an unattested Greek word, patagos, as its earliest ancestor, which perhaps meant a crash or clattering sound. That suggests that those Roman tunics might have been fringed not just with golden thread, but with solid metal adornments that may have noticeably clattered together when stirred.