Ki is a term used in the hit anime Dragonball, Dragonball Z and Dragonball GT that measures someone’s power level. It is a basic reading of how powerful a fighter is. Saiyans have the ability sense a person’s Ki, and there are also scanners that you will see Frieza and his henchmen using that can also read someone’s Ki. Ki is used for attacks and other abilities that fighters use. The higher Ki you have the more abilities and attacks you are capable of using. For a Saiyan, a high enough Ki can allow them to reach Super Saiyan levels.