Thin Armor (5e Variant Rule)
From D&D Wiki
At your DM's option, you may purchase or start out with a set of thin armor instead of normal armor. Thin armor can be applied to any armor that is Medium or Heavy.
Medium thin armor is 3 lb lighter. If it gives you Disadvantage on Stealth, it no longer does. The base armor class is reduced by 1. For example, if it is normally 13+Dex (maximum 2), then it would be 12+Dex (maximum 2).
Heavy thin armor is 5 lb lighter. You cannot gain Disadvantage on Stealth by wearing Heavy thin armor. The base armor class is reduced by 1. For example, is it is normally 17, then it would be 16. If you have proficiency in Medium armor, you count as having proficiency with Heavy thin armor.
All thin armor is 2 times as expensive as the normal equivalent, because of the skilled labor required to make effective armor with less material.