Complex sentence: In the Victorian era when the term "mail" was used to any form of metallic body armour; modern historians reserve the term "mail" for chain mail and its varieties-specifically an interlinked mesh of metal rings.
Split into simpler sentences: In the Victorian era the term "mail" was used for any form of metallic body armour. Modern historians reserve the term "mail" for chain mail and its varieties, specifically an interlinked mesh of metal rings.