Ok, here we go.
Before anyone else comments on this, i will tell you that you have asked a loaded question with this post.
Before i tell you what the general "believers" definition is, i will tell you that there are believers and non-believers of Chi, or Ki. Some people think that chi is NOT, and is fake or non-existent.
Then you get the people that think that they can make in-animate objects move, or levitate... you get my drift.
OK, what is CHI? Chi is "life force" or "electricity" in the body and all living objects. The reason Chi has so many definitions is because there is no one word in the english dictionary that fully translates from the Chinese or Japanese words for Chi. If you want to read more about chi, look up a Chi-gung, or Chinese Medicine healing book, they can explain it pretty good.
I guess the best thing to do would be to listen to your instructor and experience Chi or Ki for yourself. Oh, and don't feel put off if someone tells you that chi is BS or nonsense and you are an idiot for believing it.
There are two sides to every story, choose your side.