A thesis statement is one sentence that says it all, puts forth the whole point of the article, especially in a persuasive paper. Some use the definition "main point," but I like the phrase "main claim" because it rhymes, though imperfectly.
But we also must acknowledge that professional writers sometimes do not have one sentence that acts like a thesis statement. Instead, the IDEA of a thesis statement is implied, usually in the first paragraph.
Students should learn to identify both a thesis statement and the idea of a thesis statement. I prefer seeing a strong thesis statement in student writing--this is one of those rules that can be broken with experience. I, however, encourage you to attempt the idea of a thesis if possible, but it's not easy.