Haki

Haki (覇気) is a mysterious concept that only a few characters can use. It appears to be ability of sorts that channels one's Spirit to intimidate opponents, However, its true nature has yet to be determined or stated. There are special types of Haki that only a certain group of "chosen ones" are said to possess.

About
It was first seen when Shanks used it against 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 when Blackbeard was commenting Luffy's bounty in Jaya, but it is only during the Sabaody Archipelago arc that the concept became more clear. Haki can be used to attack directly ignoring the power of a Devil Fruit such as in the case of Luffy feeling pain when attacked by Sandersonia and Marigold. Others examples are when Silvers Rayleigh used his sword to block Admiral Kizaru, whose Devil Fruit power is a Logia type (which usually means that no physical attack can touch him). It also seems that those skilled at using Haki can use it to enhance their attacks, as it was mentioned that the Kuja Pirates' arrows are imbued with Haki. The spectators were also shocked that Luffy defeated Bacura with a single punch without any Haki, and just pure brute force. It was finally confirmed by Smoker that Haki can be used to attack Logia Devil Fruits users when Hancock struck him and said it was due to her Kuja Haki.

Usage
When first seen, it was used to invoke fear into Lord of the Coast, causing it to flee in terror. Although, Haki can be used for both offense and defense in combat. Emitting it directly to someone can destabilize an opponent; and animals seem to be more susceptible to its effects than average people. The noted most powerful usage seen so far is the ability to knock out someone else or a large group. While inexperienced users are restricted to intimidating one being or blindly knocking out any with weak wills around them, those with more expertise can focus Haki into physical attacks or even pick out individuals in a large group to knock out.

Known Users
So far the characters whom have clearly shown the use of Haki are:
 * Red Haired Shanks
 * Monkey D. Luffy
 * Silvers Rayleigh
 * Kuja tribe

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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" and "aspiration" to name a few.

Though Haoushoku is translatable to "King", it refers to a "warring king" or "conqueror", one who rules by power and conquest. The script translator Stephen uses "Overlord" as its translation.

In the recent simulcast episodes by Funimation, they have chosen to use the term "Haki energy" despite official Funimation translators wanting to use the term "ambition." This was apparently Toei's decision perhaps based on what the Viz Manga has decided to use. It is not clear what term will be used in the Funimation dub or if the DVD releases will contain this term, but "Haki energy" appears to be what they are using currently.

Trivia

 * Haki appears to be and is speculated to be similar in principle to common types of Chi found in various other Anime and Manga series. However, the similarities haven't been confirmed yet and may all be coincidence.
 * The ability to "read" an opponent's attack is similar to Mantra as well its general description. Since the Sky Island is very isolated, it is unknown if Mantra is the same type of power.
 * Jinbei as well as Whitebeard and some of his division captains have shown the capability to strike Logia Devil Fruit users. It is unknown if they can utilize Haki, or did it through some yet unknown method.
 * In the anime, Luffy implied that Marigold and Sandersonia's Haki-powered blows were similar to Sentoumaru's. However, as of yet, he is not a confirmed Haki user.