The definition of Web 2.0 has always been the subject of debate. Part of this could well be because there is no short and sweet definition. Tim O'Reilly is recognized at the leading light in this area but I have trouble remembering all the aspects of even the compact definition.
Then I think back to the Web 2.0 Expo in San Francisco in early 2007. The conference T-Shirt was a write your own "Web 2.0 is..." We then had about 3 inches of white space to write the answer. Yes - THREE Inches! Now that's a challenge! After all Tim's compact definition is about 3 column inches of newsprint!
Well, for those of you that want a really compact definition try these over at CafePress.