Haki

Haki (覇気) is a form of spirit energy of sorts that a few notable characters use. The way it is common with the Kuja suggests it theorically could be used by anyone who learn it, although there are special types of Haki that only a certain group of "chosen one" can use.

Usage
Haki is a form of energy that can be used in combat, emitting it directly to someone can destabilize an opponent, as well to be used in attacks or weapons.

It can be used to knock out someone else or a large group (although someone prepared can resist to it), predict someone action or neutralize a Devil Fruit power.

It was originally seen in the first chapter when Shanks used it against the Lord of the Coast in an attempt to save a young Luffy from the wrath of the Sea King. . While it was rarely seen some times later, 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, and also Silvers Rayleigh using his sword to make contact with Admiral Kizaru whose devil fruit power is Logia type.

So far the characters whom have clearly shown the use of Haki being Red Haired Shanks, Monkey D. Luffy , Silvers Rayleigh , the Kuja. The way how Monkey D. Garp and Sentoumaru could damage Luffy with physical attacks suggests the usage of Haki. It is expected that various other characters in the series who possesses enormous power are Haki users.

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 unknow if Mantra being the same type of power.

Levels

 * Haoushoku - Also known as the "King's Disposition". This is the Haki level achieved by a select chosen few (one in a million to be precise). This 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 and doesn't even know about it) and Boa Hancock (who hasn't mastered it). Silvers Rayleigh and Shanks may have it as well because they also have the ability to make people faint with "will power". This level of Haki doesn't require physical contact to be released, and that makes it so dreadful, such as Luffy's ability to cause a great number of the Kuja to faint.

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.

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.