Thesis comes from the Greek θέσις, which means "stance", or "position". A thesis is also itself a negativity, according to Hegelian dialectic, which means that it is not at all a "stance", but a set up for a counterpart of itself to be compared to.
A general conception of a thesis would be "A statement or theory that is put forward as a premise to be maintained or proved", but already in this statement, it shows that a thesis is already something set; a premise to be maintained or proved already positions the thesis to be in the state of longing for a presupposed, pre-determined conclusion. That is to say, "Read the thesis of this document and that is all there is. The rest of the paper is for support only." Such is a contradiction (moreover: an oxymoron) to what an essay should be.
The traditional "essay" (the meaning is still retained in today's world) comes from the French word "essayer", which means "to attempt to", or "to try to". Therefore, being used as a verb, if I essay something, I am providing an attempt to have something to do with that thing; in other words, to play with it. To write an essay on something, as it logically follows, is to provide an attempt to play with that thing, as what a child does with his first encounter with a toy. Without difficulty, I dare myself to ask: what is the role of a thesis in an essay, then?
I can never get the true meaning of "thesis", and truthfully, I have a major contempt for such terms. If there's anything worse than failures and lies, it is inconsistency (which is a sub-category of hypocrisy). Why do we insist students to have a thesis while what they need to do is to play with a subject?
In my view, thesis is the main argument/claim presented in the paper. If students have trouble understanding it, I sometimes ask them to summarize their essay in one sentence - that would be the thesis statement. As Deanna explained to us, thesis usually contains the issue, the comment on the issue and explains why it is important. The body paragraphs elaborate on the thesis and provide different evidence and support for it.