As I learned the term, a high-key image is one where the shadows are effectively eliminated, and the mid-tone detail pushed into brighter zones. My question is simple: is this an accurate definition, or is it too simple (or just plain wrong)?

I've seen the term used to describe photos which just happen to have a lot of light, but also have significant areas of deep shadow, and a lot of detail in the mid-tones. I've also seen it to simply mean a photo where the exposure decision is slightly higher than typical. Are these uses "within bounds"?

And, it may be obvious, but: is low-key the direct opposite of high-key, but dark instead of light, or is there something more subtle?

I don't know very much about studio lighting; I've heard that there's something called a "key light", and these terms may relate. Do they, and how? Wikipedia says so, but I'm not sure I believe it. I always assumed that the term came by analogy with music*. Is it possible that the terms "high-key lighting" and "high-key photograph" are subtly different? (That is, does high-key lighting always result in high-key photographs, and can high-key photographs be made without a specific studio lighting set-up?)

* "those songs which are made for the high key be made for more life, the other in the low key with more grauetie and staidnesse" — 16th-century Composer Thomas Morley