What defines a PhD dissertation/thesis?
Don't have a source, and things may be different in different parts of the world, but I've always considered a dissertation to be: 


  A long monograph submitted in partial completion of the requirements for a PhD 



Dissertations are generally submitted by grad students as they complete their PhDs -- other academics might publish long monographs, but that wouldn't be considered a dissertation. 
Certainly I don't think there is a well-defined standard for how significant a contribution has to be to merit a dissertation -- it's whatever the committee will accept (though in principle, it should have some new advance, not merely a survey or report).
Some institutions may allow you to staple together your papers to produce a dissertation; most require a separate document that re-hashes work that may (or may not) have been published elsewhere.