Ki is the metaphysical energy utilized by various characters in the Ed Edd n Eddy Z franchise, most notably the Saiyan cast and, to some extent later on, the anthropomorphic animals. It could easily be considered a conceptual mixture between Dragon Ball's Ki, and Naruto's Chakra concepts.
Ki is the latent energy created through the subconscious mix of the body's physical energy, and the mind's mental energy. It is noted by Goku in Episode 42, that both energies are limited by the other in that one can only go so far without the other.
Once Ki is produced through the mixing of these energies, it is channeled through the individual through a type of Ki circulatory system. To increase the rate of flow of Ki in said system is to "Power Up", and to slow it down is to suppress one's energy and battle power.
The body's Ki circulatory system is naturally suppressed by number of gates located throughout the body. It is noted that the number of points is dependent on the species, as Humans/Saiyans/Anthropomorphic Animals all have eight, while Namekians have five. Through proper training one can temporarily remove these limitations, thereby "opening the gates" allowing a massive increase in Ki output through the system.
Auras of ki radiate from the user when the flow of ki is increased within the body. It is noticeable that auras only appear to fighters who are strong enough to generate a certain level of Ki. Some techniques (such as both the Kaioken and the Galick Energy) are capable of supposedly increasing the output of Ki that is released from the body through unknown means.
Auras are also capable of displacing solid matter, as shown in the series multiple times in the form of levitating debris and damaging the landscape.
The color of ones aura is dependent of the user, to which it is usually the same color as the fighter's current Ki color.
Ki is most predominately seen being used in combat in the form of energy based attacks. The simplest form of this is even usable by those who, despite having a higher battle power, haven't truly experienced combat, as if reflexively. However, this is only seen by members of the Saiyan race.
Nearly all users of Ki within the series have shown the capacity of attacking with element-based attacks. This is due to the fact every being's Ki has a certain quality to it, which translates to a different element. Through thorough understanding of one's Ki a fighter can "transform" their Ki into a sort of elemental energy that can then be used to attack or increase their effectiveness in combat.
Certain fighters are shown to have multiple elements, such as Edd who has control of the five basic elements; Fire, Water, Wind, Earth, and Lightning.
It is also possible to mix the newly created Elemental Energy with one's own Ki to create an even more devastating attack. This is shown by Edd with his Fire Masenko attack, as well as Tai Ma with his Lightning Kamehameha.