Haki is a mysterious power that is found in every living being in the world. It is not that different from the normal senses. However, most people do not notice it or fail to awaken it. Broadly, there are two types of Haki common 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 and overpower enemies.
"Haki" is a power that lies dormant in all the world's creatures... "Presence", "fighting spirit" and "intimidation"... It is not different from the things that humans can naturally sense such as these... 'The act of not doubting'. That is strength!
— Silvers Rayleigh explaining Haki to Luffy at the start of his training.
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 (Aisa and Otohime) were born with the ability. Due to the fact that Haki originates from an individual's spirit and not their body, even if the user's spirit is transferred into another body, they are still capable of using Haki as if they were in their original body.
Types of Haki
Haki is separated into three categories, or "colors" (色,shoku?): Kenbunshoku Haki, which grants users a sixth sense of the world around them and limited precognitive abilities; Busoshoku Haki, which allows the user to use their spirit as armor to defend against attacks or to make their own attacks more potent; and 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. 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.
In the manga, those hit with Haoshoku Haki foam at the mouth after or while losing consciousness, whereas in the anime, they simply faint.
In the anime, use of Haoshoku and Busoshoku Haki creates a rippling or shockwave-like effect, similar to the Gura Gura no Mi, while in the manga, it is completely invisible. Also, in recent episodes, use of Haoshoku Haki causes the affected area, excluding the user, to gain a dark blue tint. However, this could just be for dramatic effect. In the One Piece Film: Z, the Haoshoku Haki's use creates a circular purple wave.
In the last anime episode before the time-skip, use of Busoshoku Haki as "invisible armor" was shown as a swift percussion forming a light-blue beam, similar to lightning, while in the manga, it is shown simply as an amplified impact, nothing else visible is shown.
Translation and Dub Issues
The term "Haki" is a word that has multiple meanings when translated into English. It can mean "ambition", "will power", "drive", "spirit", "vigor" and "aspiration" to name a few.
In the simulcast episodes by FUNimation, the subs provided have chosen to use the term "Haki". This is apparently Toei's decision as the subs are not created by Funimation, but are from the source itself.