I wrote my Master's Thesis, and my Ph.D. dissertation. To the schools which I attended, a thesis is a document which writes about a specialized area. The theory behind writing a thesis, is that you become the specialist of an area. Essentially, with a thesis, you are learning about an in-depth topic.
When I wrote my dissertation, this is a document in which you need to be a creator, and must create something new that the world has not seen/developed. Here, I had to become a specialist (as in the information required for a thesis), and then applied that information to create/develop/prove something new. This required much more support and testing data than a thesis.
Feel free to ask any other questions,