rerebrace

Wikipedia:
A rerebrace (sometimes known as an upper cannon) is a piece of armour designed to protect the upper arms (above the elbow). Splint rerebraces were a feature of Byzantine armour in the Early Medieval period. The rerebrace seems to have re-emerged in England, in the early 14th century. As part of the full plate armour of the Late Middle Ages and Renaissance the rerebrace was a tubular piece of armour between the shoulder defences (pauldron) and the elbow protection (couter).

A type of arm armor that is worn on one's upper arms.

Types

Single-sided - A sheet of armor plating strapped to the upper arm.
Tubular - A tube shaped armor piece that goes around the entirety of the upper armor.

Not to be confused with

armlet - This is an upper arm jewelry. Armlets have less coverage and protective value compared to a rerebrace.
spaulder - Spaulders cover the shoulders as well as the upper arm.