I did some research into terminology and armour, so for all your dearest Zontopian OC and side character needs, here are some armour terms I haven't seen around much. This list is far from exhaustive, but it does represent a few hours of research I thought I’d share with the community.
Ailette - A boiled leather or plate shoulderguard.
Aketon - Quilted garment worn under armour.
Aventail - Detachable mail hung from a helmet to protect the neck and shoulders.
Balteus - Leather belt with decorations and supplements worn under armour.
Bascinet - A conical helmet with an extended visor.
Behter/Behteret - A Persian type of plate mail.
Besagew/Besague - A rounded plate that covers the shoulder joint.
Bevor - Worn with a sallet to cover the jaw and throat.
Boss - The round or cone-shaped metal plate at the center of a shield, protecting the hand.
Brassart - Plate that covers the section of upper arm from elbow to area covered by shoulder armour.
Brigandine - Cloth garment, generally canvas or leather, lined with small steel plates.
Buffe - A lowered visor, often on a burgonet.
Burgonet - Open face bowl shaped helmet with a neck collar.
Byrnie - A mail shirt, mid-thigh length, with elbow-length sleeves.
Camail - A curtain of mail, hanging from the bottom of the helmet.
Cassis Crista - A large helmet with plume crest.
Cap-a-pied - Fullbody armour.
Cestus/Caestus - A Roman gladiatorial boxing glove.
Cervelliere - A skull cap.
Couter/Cowter - Plate that guards the elbow.
Cuirbouilli - Hardened boiled leather armour.
Cuisse - Plate that covers the thighs.
Enarmes - Leather straps used to grip a shield or buckler.
Epaulet/Epaulette - Shoulder plates.
Fasciae - Thick soft padding on legs.
Fauld - Bands to protect the front waist and hips, attached to cuirass.
Gambeson - Decorative coat armour worn over the breastplate.
Gardebras/Gardebrace - Extra plate that covers the front of the shoulder and the armpit.
Gauntlet - Gloves that cover from the fingers to the forearms.
Guard - An additional layer of armour that goes over cowter.
Gorget - Steel collar to protect the neck.
Gousset - Mail that protects areas not covered by plate.
Greathelm - A cylindrical helm with a narrow visor.
Greave - Covers the lower leg, front and back.
Guige - The strap which slings a shield from the shoulders or neck.
Harness - A general term for armour.
Haubergeon/Hauberk - Mail shirt reaching to the mid-thigh with sleeves.
Jupon - A short, fitted surcoat, worn over armour.
Kalantar - A Persian type of plate mail.
Kilt - A piece of fabric, leather, or plate wrapped around the waist covering the legs.
Lamel/Lammellar - A type of armour made from overlapping fragments.
Linothorax - A Greek type of cuirass.
Lorica Plumata - Roman body armour that looks feathered.
Mail - Armour made out of interlinked rings.
Manica - Segmented or scaled arm guard made of leather metal.
Pixane - A mail or leather collar.
Panser - Fortified armour.
Plackart - Extra layer of plate armour initially covering the belly.
Poleyn - Plate armour for the knee.
Rerebrace - Plate that covers the section of upper arm from elbow to area covered by shoulder armour.
Rivet - The sliding segments of shoulder armour.
Rondel - Any rounded covering.
Sabaton - Covers the foot.
Salade/Sallet - A tailed helmet.
Schynbald - Plate that covers only the shins.
Scutum - Roman rectangular shield.
Sight - The ‘eye slot’ in a helmet’s visor.
Skullcap - A bowl helmet that protects the skull.
Solleret - A plated shoe.
Spangenhelm - A plated helm, with or without jaw guards.
Spaulder - Bands of plate that cover the shoulder and part of upper arm.
Standard - A mail or leather collar.
Surcoat - A long, tunic-like, cloth garment worn over the armour.
Tabard - A piece of cloth worn over armour that covers the front and back.
Targe - A small wooden shield with covering.
Target - A small round shield mounted on the arm.
Tasset - Bands hanging from faulds or breastplate to protect the upper legs.
Umbo - The round or cone-shaped metal plate at the center of a shield, protecting the hand.
Vambrace - Forearm guard.
Void/Voider - Underarm protection.
Yushman - A Persian type of plate mail.
Zischägge - A segment-tailed helmet.
May not follow the naming conventions, but could come in clutch in certain circumstances.
Kusari zukin - Mail coif.
Kote - Vambrace and lower pauldron.
Kusazuri - Fauld or tasset.
Kusari katabira - Hauberk.
Kikko katabira - Brigandine.
Kusari shikoro - Aventail.
Armet - A closed, visored bowl helmet.
Barbute - A closed helmet with a T- or Y-shaped visor.
Buckler - A small shield.
Cullet/Coullet - Ass plate.
Cuirass - Covers the chest, not the back.
Hundsgugel/Houndskull - A conical helmet with an extended visor, also bascinet.
Jack/Jacques - A cheap defensive coat of fabric or leather.
Manifer - A jousting gauntlet.
Mantle - Covers the shoulders, collarbones, and upper back.
Morion - A helmet with a strongly curved brim and high “comb” on top.
Passegarde/Pasgard - Elbow guard/neck guard.
Pauldron - Covers the shoulder.
Piccadill - A protective collar.
Shield - You know what a shield is.
Set/Garniture - A set of armour.
https://swordsandarmor.wordpress.com/2010/09/07/medieval-armor-glossary-and-terms/
https://en.wikipedia.org/wiki/List_of_medieval_armour_components
https://www.medievalcollectibles.com/glossary/terminology/armor-terminology/
https://en.wikipedia.org/wiki/Murmillo
https://www.researchgate.net/publication/329033725_Muscovy's_Armour_Terminology_1445-1625_in_Russian
