plastrum

Noun

plastron (plural plastrons)

1. The nearly flat part of the shell structure of a tortoise or other animal, similar in composition to the carapace.
   • The great tortoise that in myth is said to support the world and is thus an embodiment of the galaxy, without whose swirling order we would be a lonely wanderer in space, is supposed to have revealed in ancient times the Universal Rule, since lost, by which one might always be sure of acting...1982, Gene Wolfe, chapter 28, in The Sword of the Lictor (The Book of the New Sun; 3), New York: Timescape, →ISBN, pages 208–209
2. A half-jacket worn under the jacket for padding or for safety.
3. A man's shirt-bosom.
4. An ornamental front panel on a woman's bodice.
   • I bought here a wedding dress perhaps twenty or thirty years old [...] a sequin plastron to be worn over the womb as a feminine equivalent to a cod-piece, and a gauze veil embroidered in purple and gold.1942, Rebecca West, Black Lamb and Grey Falcon, Canongate, published 2006, page 784
5. A breastplate.
   • "Why, compared to what thou wast, Hugo, thou art as a deerskin coat to a steel plastron.-- Art thou not in love, man? Answer me!" "Something like, I fear me, beau sire,” replied the squire.1831, George Payne Rainsford James, Philip Augustus, Or, The Brothers in Arms, page 122
6. A film of air trapped by specialized hairs against the body of an aquatic insect, and which acts as an external gill.
   • The plastron of a diving beetle is not directly a source of oxygen, but acts as a gill, acquiring oxygen from the surrounding water.