How is that part of an armor that protects the arm called, again? Pro tip: It is not an arm plate. If you work as a game developer or concept artist and are working on a Middle Age or Phantasy-themed Project, it is important to get to know the anatomy of real, historical armors and learn the name of their parts as well. The complete set of plate armor, for example, is a body harness (in Spanish: arnés, in Portuguese: arnês).
In the following link, you will have access to a short glossary of 23 entries, containing the names of the most common pieces in Middle Ages and Renaissance plate armors, in English, Portuguese and Spanish: