I've decided to be pedantic today, so here you go.
There are differences between Master Project, Dissertation and Thesis. I have to write a dissertation for my masters and a thesis for my PhD.
~ A master/major project (I'm guessing on this one as we don't have them here) is a major body of research, though not necessarily based in original research...
~ A dissertation is a longer work, for us it's 15,000 words (~60 pages double spaced) for the MLitt or 40,000 words (~160 pages double spaced) for the MPhil. A master's dissertation IS expected to be based in original research and make a new contribution to the understanding of the topic.
In the states, I think a normal MA or MS dissertation is usually around 60-70 pages.
~ The PhD Thesis is typically 80,000 words (~320 pages double spaced) and should make an original and significant contribution to knowledge.
I think a lot of the confusion over nomenclature dervives from the fact that it's not standardised between universities.
~ShyViolet