Obviously the meaning of words change, but in scholarly discussion (and that's all that I'm referring to - I don't think it really matters what words people use in everyday life to refer to particular kinds of armour) meaning can, and for the most part must, be fixed in order that authors may be able to discuss a topic in a mutually intelligible way. I'm not sure I understand your second point, nor have I ever heard mail referred to as "chain" in anything but slang.
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