Haki/Busoshoku Haki

Busoshoku Haki allows the user to create a force similar to an invisible armor around themselves. Similar to Tekkai, this Haki allows the user to defend against attacks that would otherwise cause them harm. Significant physical force can still overcome this defense; for example, Luffy struck Boa Marigold hard enough with Gear Second techniques that it sent her reeling.

Background
It was used by Garp for the first time against Luffy to bypass his Devil Fruit power and manage to  hurt him in Water 7. Later, Sentomaru used a haki-imbued attack against Luffy and which made the Straw Hats think he was a Devil Fruit user because of lack of knowledge about the ability. Later Rayleigh used Haki to stop Kizaru and touched his Logia body. First time, Kujas named this ability "Haki" while using haki-imbued arrows. In the Marineford War, Haki was used so many times by marines and pirates. It was shown that Haki is really common in the New World unlike in paradise where haki users exist merely and nearly nobody knows about it. It is explained by Rayleigh when he started to train Luffy. In fact, it seems to be one of the main requisites to have learned (especially) this type of Haki to counter effect the Devil Fruit abilities or possess enough battle power, in a simlar way to the be a Devil Fruit user when entering into the Grand Line, something that is quite unusual in the four Blues.

Usage
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 Kuja'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 Devil Fruit user, touching the "substantial body" beneath whatever protection the fruit provides, such as the case of body-altering Devil Fruits such as Logia or Paramecia users. However, unlike seastone, it does not negate a user's Devil Fruit powers, allowing them to defend themselves, and continue fighting normally. By the same effect, this Haki can also be used as a higher level of offensive, more so than just bypassing a foe's Devil Fruit powers.

A heavy concentration (or coating) of this Haki can be used to harden parts of one's body, turning the area black, thus making the user denser and more formidable, and giving their attacks greater power. This was first used by Luffy to harden his arm and later his forehead, a technique he called Busoshoku: Koka (武装色 硬化). This technique was not seen again until Vergo used it on Punk Hazard, followed shortly thereafter by Smoker.

Busoshoku Haki works independently of the user's Devil Fruit, as shown when Luffy 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 or any other energy/non-solid attacks based on Devil Fruits). So, those who attack with parts of their body can combine their Haki and Devil Fruit powers.

Drawbacks
Apparently, Akainu has found some way to avoid Haki attacks, as he was not visibly harmed even after Marco and Vista sliced him in the neck with Haki infused attacks. It is unknown how he can protect himself from Haki-imbued attacks but when he was attacked by Whitebeard with this type of Haki, he could not protect himself and he bled.

Later, although Vergo is imbued his whole body with hardened Haki which nullifies powers of Devil Fruits, Law bypassed his Haki and bifurcated him easily by using his Devil Fruit technique. It is unknown if Law used Haki to nullify Vergo's Haki but it is similar to how Akainu was able to nullify it.

Techniques

 * Busoshoku: Koka (武装色 硬化): This technique somehow uses Busoshoku Haki to harden the body (or parts of it) and object which the user is holding. It was first seen used by Monkey D. Luffy, in combination with Gear Third, in order to use his "Gomu Gomu no Elephant Gun" against the Kraken. When a body, body part, or object is hardened, it becomes black and has the same or stronger effect than the invisible application of Color of Armaments. It is implied that Hardening is stronger than the invisible application as the characters who are able to use Hardening are not seen utilizing the invisible application. When both hardened fists clash, it release massive shockwave and black-lightning like effects. So far Luffy, Vergo, Smoker, Zephyr, Chinjao,Garp and Sabo have been seen using it. Zephyr had earned the nickname "Black Arm" because of his mastery of it. It can also be imbued into weapons, as Vergo utilized it on a bamboo stick, and Smoker on his Nanashaku Jitte, the latter indicating Busoshoku Haki can be used in conjuncture with Kairoseki. Also called Buso Koka (武装 硬化) by Luffy when he uses advanced form of "Gomu Gomu no Gatling". In the Viz Manga, it is called Color of Arms Hardening and FUNimation subs, it is called Armament: Hardening.
 * Atama Buso (頭武装): Used by Luffy, he imbues his forehead with Busoshoku: Koka to harden it. This is first used in preparation for a Gomu Gomu no Kane against the Iron Shell Squadron. In the FUNimation subs, it is called Head: Hardening.
 * Gear Second Buso (ギア2（セカンド）武装): Used by Luffy, he imbues his fist with Busoshoku: Koka while under the effect of Gear Second to harden it, which also seems to have the added effect of igniting it. It was first used in preparation for Gomu Gomu no Red Hawk against Hody Jones. In the FUNimation subs it is called Second Gear: Armament.

Translation and Dub Issues
The Kōka in Busōshoku: Kōka can also translate as "vulcanization", which is the process of rubber (or similar substances) being converted into a denser material.

Trivia

 * Busoshoku Haki is similar to Tekkai. The difference between them is that Tekkai depends on the user's physical strength, but Busoshoku Haki depends on the user's willpower.
 * Busoshoku Haki users can still utilize their Devil Fruit powers, despite Haki being able to negate Devil Fruit abilities. This is because Haki only allows the user to bypass defenses provided by other peoples' Devil Fruit powers. In fact, Devil Fruit users can use the two abilities together to make one or the other more effective.
 * Busoshoku Haki is the most used type of Haki in the series so far.
 * Although the black skin effect of its usage seems to be pretty ordinary when used at strong levels, it was seen only after the time skip. Several powerful characters (Rayleigh, Marine Admirals and Vice-Admirals, Whitebeard Pirates, Jinbe, etc.) have been seen using Busoshoku Haki before the time skip, but the black skin effect was not shown (this can be noted especially during the Summit War). The reason of this could be that Oda simply decided to use it after the time skip, (especially to demonstrate visually the use of Haki of his main characters, like Luffy, who before the time skip could not use this type of Haki) or he did not get the idea until that time.