Final answer:
The best term for a one-sentence declaration that states an article's argument is a 'thesis statement.' It's crucial in an essay as it outlines the main argument and guides the reader through the writer's perspective.
Step-by-step explanation:
The term thesis statement best describes the one-sentence declaration that delineates what the argument in an article or an essay will be. A thesis statement is crucial for outlining the main argument and serves as a guide for the reader to understand the writer's position and the direction of the paper. It is made up of a fact, the writer's opinion, and the reasoning behind the argument, and it is typically found at the end of the introduction paragraph.
While forming a thesis statement, it's important to keep it short, specific, debatable, and clear. This will make it easier for the audience to comprehend the position you are taking on a debatable issue. The rest of the paper will support this statement with credible evidence and a clear structure, whether that be a thesis-first or thesis-last organization.