Mithral Armor

From Dungeon Master's Guide, page 150. Found on Magic Item Table B.

Description
Armor (medium or heavy, but not hide), uncommon.

Mithral is a light, flexible metal. A mithral chain shirt or breastplate can be worn under normal clothes. If the armor normally imposes disadvantage on Dexterity (Stealth) checks or has a Strength requirement, the mithral version of the armor doesn't.