Haki

Haki (覇気) is a form of spirit energy of sorts that only a few characters can use. However, the fact that it's common with the Kuja to have this power suggests it theoretically could be used by anyone who learn it. However, there are special types of Haki that only a certain group of "chosen ones" can use.

About
It was originally seen in the first chapter when Shanks used it against the Lord of the Coast in an attempt to save a 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 was 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 apparently is similar in principle to common trope of Chi or Ki found in various other Anime and Manga series such as Dragonball with Ki, Bleach with Reiatsu, and etc.
 * 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. However, it may possibly be a branch ability brought into the Sky Islands by those from the blue sea as Mantra seems to be involuntary.
 * From what we have seen, Haki might also be related to Seimei Kikan, as Sandersonia and Marigold have been seen to control their hair using Haki, in the very same way Kumadori uses Seimei Kikan.