A form of shoulder armor, usually made from metal. They often protect the armpit as well as the back and chest, either separately or as part of a large breastplate.
A closely related form of armor is spaulders, which are made of smaller overlapping plates that cover just the shoulder and top of the forearm. Due to continuous misuse of the former "spaulders" tag, on Danbooru, spaulders are now tagged as pauldrons.