Busō-shoku no Haki
Busoshoku Haki (武装色の覇気, Busō-shoku no Haki Lit. Haki of the Color of Armaments) is one of the three types of Haki.
Busoshoku Haki allows the user to create a force similar to an invisible armor around themselves. Similar to Tekkai, this allows the user to defend against attacks that would otherwise cause them harm. Only significant physical force can overcome this defense; for example, Hugo striking Boa Marigold hard enough with Gear Second techniques to send her reeling.
Naturally this "invisible armor" can be used as a weapon to strike others with. It can be used to augment the user's own attacks, projected ahead of the user as pure brute force, or imbued in weapons to increase their effectiveness. Any attack enhanced by Busoshoku Haki hits immensely harder than it would normally be without it. The Mermaid Heel's Haki-imbued arrows, for example, are capable of penetrating solid stone.
This type of Haki also has the ability to bypass the powers of a Magic that turns their body into water, fire, mud, ect., touching the "substantial body" beneath whatever protection the magic provides, such as the case of body-altering magic. However, it doesn't negate a user's body alteration powers, allowing them to defend themselves, and continue fighting normally
Busoshoku Haki works independently of the user's Magic, as shown when Hugo uses it to enhance his Gear Third attack underwater. This type of Haki may not be equipped to attacks that are not from one's body except for weapons, or anything generated from one's own body (like Kizaru's laser beams, Marco's flames). So, those who attack with parts of their body can combine their Haki and Magic powers.