plastron (noun) /ˈplæstɹən/
1
The nearly flat part of the shell structure of a tortoise or other animal, similar in composition to the carapace.
2
(fencing) A half-jacket worn under the jacket for padding or for safety.
3
A man's shirt-bosom.
4
A breastplate.
5
A film of air trapped by specialized hairs against the body of an aquatic insect, and which acts as an external gill.