What defines a PhD dissertation/thesis? This might be a bit of an abstract question, but what defines a dissertation? Some colleagues and myself have been debating this and some are arguing that three peer-reviewed publications or a long monograph make a dissertation. However, others are arguing that the dissertation is defined not by the length or number of publications, but the significance of the contribution. Is there any scholarly consensus on this, or is this a continuing discussion in academia?
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.