What is a thesis?
A thesis is 'a proposition to be discussed and proved or to be maintained against objections' (Oxford dictionary 2004). The whole thesis needs to be focused around one major idea for which you provide evidence and can defend in discussion with other scholars.
http://resource.unisa.edu.au/mod/book/view.php?id=10112&chapterid=4106
Actual entry in the Concise Oxford Dictionary, 1982
Proposition to be maintained or proved; Dissertation esp. by candidate for degree
