Origins, etymology and definiton
Ki is the Japanese (Romaji) version of Qi (or Ch'i); a term most oftenly used in our context to define the internal energy of man, animal, and nearly everything. From hereon, Ki also refers to Qi and Chi, since they are essentially the same, just different spellings of the same principle.
Although there are still some differences in the actual interpretation of what Ki actually is, on a few things we have agreed. Since we focus mostly on it's use and application in the human body, I will go from that point of view. Ki is the energy that flows inside all of us. Your Ki was there when you were born, and it will be there when you die. Without Ki, there would be nothing; not even darkness.
We know that the concept originated from China, but there is very little know about the actual origins of the concept of Ki. One of the earliest and best documented references to Ki is in the well-known treatise on TCM, the Nèijing: Medical Canon of the Yellow Emperor - attributed to the Yellow Emperor Huang Di and written somewhere between 2700 and 2500 B.C. (although many historians do believe that the actual text was compiled much later). Ki itself, however, has been mentioned since the very first 'written' records of Chinese society.
In etymology; the (older) Kanji for Ki comes from a metaphor, namely 'steam rising from boiling water' - this in term referring to the transformation of boiling water; first it's completely still, but slowly a vapor arises from the water, only to eventually turn into a powerful force that bubbles from the water, to the top and into the atmosphere. The usage of the word Ki is quite widespread; a lot of things are indicated with Ki. "I have Ki for you", for instance, means that you really like the person. And of course, it is also a common term in martial arts; an example that is almost too easy would be Aikido, an art that relies heavily on the coördination and training of one's Ki.
The way of healing, Master Lam Kam Chuen, Gaia Books, London, 1999 (2nd print, 2002)
Aikido and the dynamic circle, A. Westbrook/O. Ratti, Charles E. Tuttle comp., Japan (47th print, 1998)