Ki is also known as "latent energy" or "fighting power." The term is the also known as Chi, which directly translates as "Life Force." This force is a tangible energy inside a living being, it's major focus being in the center of the body. By drawing it out, a person is able to manipulate it and use it for perfomances outside the body. Ki can be used for many different techniques. Usually the more concentrated the masses are, the more time the user requires to draw it out (to "power up"). When a fighter gathers Ki, he is able to gain enhanced strenght, speed, endurance, and can increase the power of their attacks to inflict greater damage to opponents.
Just like Aura , Spiritual Energy, and Chakra , Ki come's in a variety of color's. The most common color's in the seriesis white: other colors include yellow, red, purple, orange, blue, geen, pink, and black. These color's vary on the race, techinque being used, species that the character is or how (im)pure the being's heart is.
The term "Ki was first shown in Chapter 57 of Season 1 and then explained in Chapter 58 of Season 2 by Timothy . Timothy explain's that there was somebody before that used Ki, but their name is unknown. Timothy explain's that it was him/her that discovered this inner energy in the Pokemon DX timeline, but the first usage of it is seen when Timothy's lost control of his power's and went on a rampage firing a blast at Hinta. By drawing his ki energy into the palms of his hand, Timothy was able to expel an explosive mass of visible ki energy.