1.NounA formal exposition of a subject, especially a research paper that students write in order to complete the requirements for a doctoral degree in the US and a non-doctoral degree in the UK; a thesis.
"write a dissertation; write up a dissertation; hand in a dissertation; complete a dissertation; She spent three years writing her doctoral dissertation.; The student defended his dissertation successfully.; Her dissertation focused on medieval literature."
2.NounA lengthy lecture on a subject; a treatise; a discourse; a sermon.