Ki is a Japanese character. Let's talk about ideographic languages for a minute. An ideographic language consists of symbols or ideograms which represent concepts indepdendent of their sound. China is a grand example of this. The various dialects of Chinese actually are quite divergent of each other and an analog might well be to refer to Russian, Sanskrit, Irish and English as dialects of European. In China, the character system permits you to write a note in your local dialect and send it to any other Chinese speaker, who will render it correctly in the loal dialect.
During the T'ang dynasty, the Japanese even went so far as to create a very specific form of Japanese that corresponded more closely to Chinese, so that for a time, a letter written in Chinese could be read largely unaided by someone in Japan. Because of this, there is a very specific classical Japanese reading of characters that appears especially in things that are ancient or purport to be. A good example is the venerable style of shorinji kempo which is the Japanese pronunciation of shaolin ch'uan fa. This resonates with tradition to Japanese ears.
Part of the joy and consternation of reading an ideographic language is that there are many characters with similar meanings. Why these were used can convey quite a bit of other information. It is possible, for instance, to write a sentence that is similar to a optical illusion: There are many possibly conflicting meanings available at once. Because of this, the very Western idea of having pairs of antonyms doesn't really work, and a Chinese friend of mine said that learning such pairs was the most difficult conceptual part of learning English.
Ki is one such character. It has many, many meanings and not all of them are distinct or even complimentary. This makes learning its meanings a decidedly non-linear undertaking.
Mind you, I don't make this definition lightly. Having observed (as a geek) people who are
very good at martial arts for several years it only gradually become obvious what I needed to
focus on to reproduce what they did. So, here is where I go out on a limb:
When learning basic techniques, ki is effectively the same as structure.
As I said, at the higher levels, this is far too simple, but I assume that you need a starting point.
There are many different strategies for this. There is the simple mechanical way (locking) as well as displacing someone's hips so they have to rely on you for their structure, timing it so their structure is in the wrong alignment for what they want to do or simply pulling then leading in such a way as to keep uke from using his hips. Aikido focuses largely on this last way, which takes a lot of technical savvy as well as timing. This is why aikido is so elegant looking and takes much longer to master than other aiki-based systems.
Ultimately you learn to use the word ki in the way that is traditional. At least part of this is suggested on conservative grounds: It's traditional. Another part of this is that the multiple meanings allow you to organize this concept in a particularly Japanese way, which I think adds a lot to learning a system. You may be a gaijin (foreigner) like me and keep all the various concepts nicely compartmentalized too.
Personally, I feel that every Oriental concept has a Western explanation and I should be able to ennunciate them both.