Wikipedia
Couter
The couter (also spelled "cowter") is the defense for the elbow in a piece of plate armour. Initially just a curved piece of metal, as plate armor progressed the couter became an articulated joint.
In fighting reenactment groups such as the Society for Creative Anachronism, a couter/cowter is often called an elbow cop.
Usage examples of "couter".
This was his body: the gorget at his throat, the gauntlet, vambrace, couter of his powerful arms, the bulging pauldron at his shoulders, the cuisse and greave of his legs, the taloned claws of his monstrous feet, all his flesh and scale.
Your couter on that arm is overlarge, and your pauldron too small for a man of your shoulder.