What is "green"?

I hear about things being "green" or that are going "green" all of the time.  "This product is green", "this form of power is green", and "these are manufactured in a green plant", are part of daily conversation, or at least advertisement.  But what does "green" really mean?  It has many meanings and can be interpreted in many different ways.  Most of the time it is thought of as being good for the environment, but how good for the environment does something have to be to be considered "green"?  Does it just have to have just a few eco-friendly aspects or can it have absolutely no negative impact on the environment.  One could be so set in the definition as to say that nothing made by man is "green".  Everything that we make is at some point transported, shape, or influenced by things which negatively affect the environment.  One could also be very loose in the definition as to say that if any product has one element that decreases its impact on the environment it is "green". I think it is interesting that so many things are labeled "green", but there seem to be no guide lines for determining what is "green".  I think that there should be guide lines so that when you are buying, researching, or just hearing about things that are "green", we know what it means exactly.