Plastron - Underarm Protector
The fencing underarm protector, also called the plastron, is a necessary piece of protective equipment in fencing. This part of the uniform is worn underneath the fencing jacket and protects the weapon-arm from any sort of hit that could penetrate the fencing jacket.
Plastron are required for all fencing competitions. Standard ones are legal for USFA tournament use and FIE rated plastrons are required for FIE events. Rules in other countries vary, so check with your local fencing club for the tournament requirements if outside the US.