Chi

Chi is a spiritual art similar to Psynergy that is practiced by the culture of Xian, on the southern region of the continent of Angara. It is seen in Golden Sun during the game's story arc that takes place along the Fuchin Temple / Mogall Forest / Xian / Lama Temple region. The most notable practitioners of Chi include Master Hama of the Lama Temple and Feh of Xian, who was her student in Chi in spite of apparently being her senior.

Differences from Ki (Psynergy)
Chi is not just another name for Psynergy (which the people of Xian know as "Ki" and know is practiced by Master Nyunpa of the Fuchin Temple). As Feh explains to Isaac in Xian, Chi is the art of harnessing the inherent, innate energy laced within one's own body and either using it to bolster one's own ability to fight in a martial-arts style, which Feh refers to as kung fu, or projecting it as an invisible-yet-tangible force that can damage objects distant from the wielder. Chi can be considered non-elemental (evidenced by the fact that the Psynergy spell that is closest to Chi, the Force Psynergy, belongs to no element).

Mastering Chi Vs. Psynergy
Using Psynergy, or Ki, can become easy to an individual that has either been born with a specific gift of Adeptness to his or her innate element or has their Adeptness directly "unlocked" by sudden and strong exposure to a Psynergy Stone (such as being hit in the head by a falling Psynergy Stone from the sky). But to an individual that does not have either luxury, being a Psynergy-wielding Adept is virtually impossible to attain. Chi, on the other hand, is apparently a more physical art that a larger proportion of the world's populace can study and learn at any time without the need for Psynergy Stones. The chief drawback to Chi in comparison to Ki is that depending on the person, the body often has limits on how much energy for Chi can be stored up and how intensely it can be expulsed, and it requires much discipline to improve one's ability to use Chi in these areas. As Feh puts it, the body has many limits, but the mind has few limits, which is why the sorcerous craft of Psynergy can potentially be so much more powerful when used by capable Adepts.

Cultural References
In Japanese philosophy, which borrows from Buddhist-Chinese philosophy, Chi is one of the five classical elements, encompassing earth and stone. This also includes the compositions of the human body: the bones and muscles, which the in-game martial art aims to strengthen.