Haki

Haki(覇気) is a form of spirit energy of sorts that a few notable characters can use. Originally first seen 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. The Sea King was forced to flee when hit by this power.

Usage
Initially, Haki appears as a strange technique that seems to frighten anyone just by looking in their eyes. While eye contact appears to be unnecessary, the locking of eyes seems crucial in order to centralize the effects. A powerful user, when using this technique, could render a large group of people (provided they are deemed as foes) to immediately lose consciousness, even without locking eyes with each and everyone of them, after getting within a certain proximity from where the user stands. However the reason they lose consciousness is because they are simply not ready (determined) or strong enough to stand up against the Spirit Energy. Shanks, Silvers Rayleigh, the Kuja, and Luffy possess this technique, with the latter only recently exhibiting it.

In Chapter 434, when Shanks went into Whitebeard's ship, he made the lesser crew members on Whitebeard's ship fall to the ground unconscious. While walking on Whitebeard's ship, Shank's spirit energy was powerful enough to damage the deck of the ship as if it was hit by a powerful force. When he clashes with Whitebeard's spear to sword, the energy split the clouds apart. Later, Luffy unintentionally uses it on Motobaro, scaring it away in a fashion oddly similar to Shanks' scaring of the Lord of the Coast, eventually causing it to faint. The latest use was by the coating mechanic that the Strawhats are looking for, Silvers Rayleigh and his display was more powerful than any other as he used it to crush an entire audience of people at a slavery auction house.

Haki can be imbued with attacks to make them stronger, as seen when the Kuja fire arrows that can blast through solid objects such as walls and the ground. It is possible that Silvers Rayleigh can use haki to make contact with logia users in their element form, but not yet confirmed.

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.