A thesis statement is the sentence or short passage in which a piece of writing states the position it will defend, as distinct from the topic it will cover. The idea belongs to composition teaching in the English-speaking academy, where it is taught as a discipline against writing that circles a subject without asserting anything. Its conventions are pedagogical: where it should appear, how explicit it should be, and whether a thesis is required at all in forms like the essay or the exploratory piece are matters on which teachers and handbooks differ, and some traditions dispense with the term entirely. The familiar placement in an opening paragraph is a habit of instruction, not a rule.
Why this entry carries no source list. This lexicon cites an official primary source wherever one exists, and says so plainly where none does. No body governs this term. There is no standards organisation for it, no register, and no text whose authors could claim to have settled its meaning. Methods literature describes practice and argues about it; a citation to one author would dress a position in a live argument as a rule, which would mislead more than silence does. What follows is the convention as it is actually used, with the disagreements named rather than smoothed over.