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.
Mithral Armor in the 2024 rules
The 2024 Dungeon Master’s Guide revised the magic items, and the SRD 5.2 carries the new text. What changed:
Same rarity and attunement; the text is reworded for the 2024 rules glossary.
Type
Armor (Any Medium or Heavy, Except Hide Armor)
Rarity
Uncommon
Attunement
Not required
Mithral is a light, flexible metal. Armor made of this substance 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.