Haki

Haki can be classified as a form of fighting style for the layman in this vast shinobi dominated realm. The form of fighting style was created by Rayleigh Nara and taught to the pirates under his command during his lifetime. With the likes of Chakra being such a widespread phenomenon, Rayleigh wanted to give the average man a fighting chance with the energy that laid dormant within them all.

Focusing on the individual Physical and Spiritual Energies within, Rayleigh would go along with training his pirates to be on par with the average shinobi/kunoichi. Obviously, getting the others to cooperate with him was difficult during the beginning of his forced employment as a pirate. So, the young Nara would spend a year (or so) to hone his skill in the art before giving it a try once again.

Usage
Haki is separated in three different categories or colors (色 shoku): Kenbunshoku Haki, which uses one's Spiritual Energy to gain a precognitive (sixth sense) of the world around them by a maximum of 50 meters (for the average civilian user - 500 meters for average shinobi). Busoshoku Haki, which uses one's Physical Energy to form an invisible armor around themselves to defend against attacks or to make their own attacks more potent (level of enhanced durability varies between users). Finally Haoshoku Haki, the hardest to learn, deals with using both the energies at the same time impose one's will on an opponent.