Ki is the Japanese word for the Chinese concept of Chi. Ki has become deeply embedded in the Japanese language and many Japanese words embody Ki. Ki involves the concept of a life force that underlies all of life and perhaps all of existence. It is probably the same as the Western concept of Vitality and Vitalism. Acupuncture is based on Ki. At a basic level Ki is like electricity in that it is a unifying concept for many observed phenomena. We have no idea what Ki is just as we have no idea what electricity is. Some people think Ki is a magnetic phenomena. Ki is used to explain Acupuncture and some of Dim Mak. Dim Mak is using Acupuncture points and other anatomical points if fighting.
The point of view here is that Ki is used inappropriately to explain many things. Unbendable Arm and Circle of Iron are two examples.