As the wikipedia article mentions, psychokinesis has never been demonstrated. It's likely all a fantasy. The association of the concept of ki with this kind of mystical nonesense is one reason I would avoid using the term when explaining anything using the English language.
As others have pointed out, the concept of ki taken outside of a specific context is far too broad to associate with one specific anything.
As for the specific concepts of ki and aiki within aikido, I'm not convinced that there is one "orthodox" take on it. There seems to be an association with internal body training (Tohei's ki exercises, aiki according to Daito-ryu ), but even there I don't think the specific definition of aiki in Daito-ryu (which seems to correspond with very specific body skills according to Internal training guys with Daito-ryu connections that post here) necessarily invalidates other takes on the term within the context of aikido. As a side note, the fact that the IT crowd here can actually discuss the ki and aiki strength without calling on mystical energies is a large part of why I don't ignore them (which I am more than happy to do with the no touch throw, shooting ki across the room types).
As for the difficulties of translating ki and ki related concepts. I think everybody who claims any knowledge should try harder. Even if you need 20 words in English for two in Japanese in order to get the context through. With a term with such a broad array of meanings, context besomes especially important. Ki, like "spirit" in English, is not one specific thing, and the various things it can represent are not necessarily that clodely related to each other (which is why tranlations in one context may use very different terms than those in another context). I liked Ignatius' definition earlier in the thread from this point of view.
As to whethwer or not "ki" exists. Ki is a word in Japanese that covers many concepts with real meaning to people and as others have shown, is a basic root in a rather large array of Japanese words. As you may have gessed from the rest of my post, there are many usages of "ki" out there that I consider as referring to forms of energy or forces that are a bunch of mystical new-age type nonesence.