A thesis is an idea or theory that is expressed as a statement and is discussed in a logical way.
e.g. This thesis does not stand up to close inspection.
e.g. ...proponents of the thesis that computers can be programmed to do anything which a human mind does.
A thesis is a long piece of writing based on your own ideas and research that you do as part of a university degree, especially a higher degree such as a PhD.
e.g. He was awarded his PhD for a thesis on industrial robots.