from wikipedia: "The literal translation is "air", "breath", or "gas""
...all three of which mystically occur at our dojo with frightening regularity.
The kanji pictogram for ki,
, actually consists of two parts: One is the sign for air, vapor, or mist, and the other (inside of the former) is the sign for rice. Boiling rice, the main nutrition for the Chinese and Japanese.
So, it implies an energy or nourishment within the air.
I have written about ki - and examples of how to exercise it - on my website:
Whether you believe in it or not, it is a rewarding method to develop your aikido. But of course, muscles are good to have, too