Haki

Haki is a mysterious power that is found in every living being in the world. It is not that different from the typical senses. However, most people do not notice it or fail to awaken it. Broadly speaking, there are two types of Haki available to everyone, given the proper training; however, there is a third type that only a certain group of "chosen ones" are said to possess. In simple terms, Haki is an ability to sense spiritual energy (Kenbunshoku), use life force (Busoshoku) and overpower enemies (Haoshoku).

Overview
Haki, unnamed at the time, was first seen when Shanks used it to scare off the Lord of the Coast to save Luffy when he was a child from the wrath of the Sea King. The term "Haki" was first coined and used much later when Blackbeard was commenting on Luffy's bounty in Jaya, and was hinted at during Shanks' visit with Whitebeard.

The underlying concepts of Haki were introduced during the Sabaody Archipelago and Amazon Lily Arcs. The concept was clearly explained by Rayleigh to Luffy at the start of the latter's training on Rusukaina Island. Haki is dormant in every living person, but it's rare for most people to ever awaken that ability. Intense training can awaken it, as can extreme shock, such as with Coby during the Battle of Marineford. Two known people (Aisaand Otohime) were born with the ability awakened, with no known training. Due to the fact that Haki originates from an individual's spirit and not their physical body, users are still capable of using it even if their spirit is transferred into another body. Haki can also become stronger, especially when a Haki user battles stronger opponents.

Despite their great capabilities, Haki is not limitless as it can be depleted from overuse, rendering the user unable to use it for a set period while it regenerates.

Types of Haki
Haki is separated into three categories, or "colors", each has levels of usage, so that even two proficient users might not be of equal strength. Most people who can use Haki tend to have a type they are better at and as a result focus on that type. However, a person can improve their abilities with all the types of Haki, it just requires more work. Furthermore, at least two types of Haki can also be used simultaneously.
 * Kenbunshoku Haki, which grants users a sixth sense of the world around them and limited precognitive abilities. Users can hone their skills to such a degree that they can seemingly predict the future, as seen with Charlotte Katakuri. Enel used his Devil Fruit, the Goro Goro no Mi, to significantly increase the range of his detection radios.
 * Busoshoku Haki, which allows the user to use their spirit as armor to defend against attacks or to make their own attacks more potent. Users can learn to use the Koka Technique over their entire body and\or weapons. Furthermore, when clashing against another Busoshoku Haki user, the one with the lower level will feel to impact to a certain degree, ranging from swelling, to physical changes.
 * Haoshoku Haki, a rare type of Haki only one in a million can use which grants the user the ability to overpower the will of others. Users of this type of Haki are seemingly somehow ranked, as Chinjao claimed that one could become "the top of all the conquerors".

Trivia

 * Haki appears to be similar in principle to common types of Qi (Chi) found in various other anime and manga series; which is based on the concept of "qi" (or "life-force") in Chinese philosophy.
 * Coincidentally, the kanji for the "ki" in Haki is the same as the same character/hanzi used for qi/chi. By this proxy, the pinyin reading of Haki would be Baqi ("bah-chee").
 * Not only is it a Japanese term, but Haki was also the name of a Scandinavian Sea-King (a name given to powerful pirates) in Norse mythology.
 * In the anime, some animals seem to be able to learn to competently use Haki (particularly Busoshoku Haki). This has so far happened twice:
 * Alpacacino, member of the Neo Marines, was shown using Busoshoku Haki to damage Luffy;
 * The Kung-Fu Dugong captain of the Sea Animal Pirates.
 * Monkey D. Luffy, Silvers Rayleigh, Edward Newgate, and Charlotte Katakuri are so far the only people shown using all three forms of Haki.
 * Shanks also used all three types, however his use of Kenbunshoku Haki is non-canon.