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 beast. While it was rarely seen some times later, only after the Shabondy Archipelago arc the concept became more clear.

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" someone 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 - a Haki levels achieved by a select chosen few (1 in millions). 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).

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.