What is it and how can a human achieve it?
My grasp of Eastern metaphysics leaves a lot to be desired :rolleyes:, but…
IIRC, chi isn’t something that one achieves. It’s something that just is, like air or water. It’s like a Life Force, or a state of being. I guess it can be analagous to, oh I dunno, vibes or a mojo.
For example, you walk into the Susan Lawrence Dana House in Springfield, IL (rumored to be haunted), and you get an uneasy feeling. Whereas I (a Westerner) would say “There’s a bad mojo in here,” an Easterner would say “There’s bad chi in here.”
Or I could be wrong.
What is it? It is from the martial arts, and refers to a mysterious energy permeating living bodies that, when channeled properly, will heal you, give you amazing strength, allow you to resist strikes to the abdomen/neck, cause your body or that of others to become heavier or lighter, and enable you to perform all the amazing feats of martial arts.
How does a human develop it? You can’t; there is no such thing.
Or rather, there is no evidence that it exists, in that all the amazing feats of the martial arts can be explained completely by good old Western science or are simple BS.
I have a black belt (judo) and I have studied a number of other arts. Seen a lot of amazing things, but there ain’t no magic in this world.
I find a lot of the people who say different are trying to make money off me.
You didn’t think Star Wars made that up, did you? 
Wait a minute. Are you implying that anime isn’t entirely factual?! The hell? Are you going to tell me that Bladerunner was fictional as well? Where does it end?
Am I the only one who laughed out loud when Jason Scott Lee started lecturing on chi in Dragon: The Bruce Lee Story
That being said, while chi powers are pretty much bunk, (every martial artist that I’ve met agrees, including all my teachers) the word is often used, as rastahomie points out, as sort of a shorthand for will, calm, or (human)essence. Kinda like the way that some christains use “spirit.”
“This makes me nostalgic for the movie Gymkata.”
I’m an American living in Korea… and take it from me: Asians (or at least Koreans) take “Ki” very seriously…
Recently, on Korean TV, there have been several shows about people who posses/can gather large amounts of “Ki” and perform seemingly incredible stunts with it. One man, who has been on several shows, knocks over cigarettes and business cards that have been stood (standed? stooded? PLACED!!) on end. He does this by stroking his right index finger from his right ear to his chin, then from his left ear to his chin, and then rapidly whipping his hand down close to the target. The target then falls over. My fiancee (a very sexy, normally sensible, WAY smarter then me Korean woman [yes, I’m a lucky bastard!]) was really impressed by this! It was obvious to me that quickly-moving-hand produced wind was the key to knocking over cigarettes, but my fiancee didn’t buy it! Even when I demonstrated that I could do this with a cig on my desk (not as easy as it sounds, oddly, but after a try or two… no problem!)
The lesson from this? “Ki” may or may not exist… but people who are convinced that it does are easily fooled.