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. However, it's 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 in an attempt to save Luffy when he was a child from the wrath of the Sea King. . The term "haki" was first coined and used when Shanks confronted Whitebeard on his ship, but it is only during the Sabaody Archipelago arc that the concept became more clear. Haki can be used to suppress the power of the devil fruit such as in the case of Luffy feeling pain when attacked by Sandersonia and Marigold. Another example is when Silvers Rayleigh used his sword to make contact with Admiral Kizaru, whose devil fruit power is a Logia type (which usually means that no physical attack can touch him).

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. It can also be used in physical attacks or with weapons.

The noted most powerful usage seen so far is the ability to knock out someone else or a large group.

Levels

 * Haoushoku（覇王色）: Also known as the "King's Disposition". This is the Haki level achieved by a chosen few (one in a million). A person with this level of Haki is powerful enough to take out many of the Kuja warriors who are already accustomed to regular Haki. So far, the only two identified with this level of Haki are Luffy (who recently only began showing signs of this power and doesn't even know about it) and Boa Hancock.

Known Users
So far the characters whom have clearly shown the use of Haki being Red Haired Shanks, Monkey D. Luffy , Silvers Rayleigh and the Kuja.

Translation and Dub Issues
The term "Haki" is a word that has multiple meanings when translated into English. It can mean "ambition", "will power", and "aspiration" to name a few.

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

Trivia

 * Haki is speculated by fans to be similar in principle to common trope of Chi found in various other Anime and Manga series such as Dragonball with Ki, Bleach with Reiatsu, Hunter x Hunter with Ren, Naruto with Chakra, etc. However, it's all just fan speculation based on what little has been shown, as Oda hasn't confirmed the true nature of it yet.
 * 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.