how to clearly define a dissertation

What does the word ‘debate’ imply? A discussion involving different points of view or sets of ideas. A dissertation will therefore not only examine a subject but will review different points of view about that subject.
Another useful clue is found in the Latin origin of the word – dissertation comes from a Latin word ‘dissertare’ = ‘to debate’.

References:

http://zsr.wfu.edu/digital-scholarship/etd/tips-for-abstract-and-keywords/