Ki is a generic yet important term which refers to energy. It regroups 3 tied notions in the game : energy gauge, charging rate and energy attacks.
Energy gauge Edit
In this way, "Ki" is the amount of fighter's energy.
The gauge is divided into 5 segments, in order to easily define how much of energy fighter currently haves, and how much his special moves costs. For example, a Kamehameha with standard Goku costs 3 Ki bars (or 3/5 of the gauge) to perform.
Ki (or energy) is gained mainly by charging it up (by maintaining L2 button on PS2 or L on Wii), but fighter slowly raises Ki by rushing ennemy, by waiting (until a certain portion of the gauge, depending on character), and by using some Blast 1.
Obviously, fighter loses Ki/energy by using special moves, including Ki blasts, Blast moves, counter-attacks, high-speed movement, Dragon dash and so on. But a chunk of Ki is also lost when one has his guard broken by ennemy.
Building energy speed Edit
The statistic "Ki", as it is referred in Evolution Z screen, is in fact the rate at wich a fighter raises his energy (so, his Ki in the "amount" meaning).
So, the higher the Ki level, the faster the Ki regeneration. The purpose is obvious : gather energy quickly to use special moves continuously.
Some Z-items, like Ki +1 or Oolong's support, boost the Ki level. Depending on the item, the statistic is either called "Ki level" or "Energy". But the statistic in the fighter selection screen is spelled "Ki", and the effect is with no doubt the regeneration rate.
Ki-based attacks Edit
Being the energy in the Dragonball universe, Ki is used in numerous attacks. For example, Ki Blasts are small fireballs almost every fighter can throw (by tapping Triangle on PS2 or B on Wii). Energy beams (including Kamehameha) are also attacks made of Ki.
But it is purely cosmetic in the game. Ki blasts damages are determined by Attack, Blast moves damages are determined by Blast 2 and Ultimate Blast statistics. The Ki level, nor the amount of Ki, don't impact any damages, dealt or received.