Haki

Haki (覇気 Haki, literally meaning "Ambition") is a mysterious power that is found in every living being in the world. It is not that different from the normal senses. However, most people do not notice it or fail to awaken it. Broadly, there are two types of Haki common to everyone, given the proper training; however, there is a third type that only a certain group of "chosen ones" are said to possess. In simple terms, Haki is an ability to sense spiritual energy and overpower enemies.

Descriptions
Haki is dormant in every living person, but it's rare for most people to ever awaken that ability. Intense training can awaken it, as can extreme shock, such as with Coby during the Battle of Marineford. Two known people (Aisa and Otohime) were born with the ability.

Due to the fact that Haki originates from an individual's spirit and not their body, even if the user's spirit is transferred into another body, they are still capable of using Haki as if they were in their original body.

Types
Haki is separated into three categories, or "colors": Kenbunshoku Haki, which grants users a sixth sense of the world around them and limited precognitive abilities; Busoshoku Haki, which allows the user to use their spirit as armor to defend against attacks or to make their own attacks more potent; and Haoshoku Haki, a rare type of Haki only one in a million can use which grants the user the ability to overpower the will of others. Most people who can use Haki tend to have a type they're better at and as a result focus on that type. However, a person can improve their abilities with all the types of Haki, it just requires more work.