What tag defines a header for a document or section?

The tag that defines a header for a document or section is commonly known as the Heading Tag. In HTML, these tags range from H1 to H6, with H1 representing the most important heading and H6 the least important. These tags are used to organize content on a webpage, making it easier for users and search engines to understand the structure of the document. The heading tags also play a critical role in SEO, as they help search engines determine what the content is about.

Other options do not accurately represent the tags used for creating headers. While there is a "title" tag in HTML, it specifically defines the title of the document that appears in the browser tab and is not used for section headings. The options "Header Tag" and "Top Tag" are not officially recognized HTML tags for headers, leading to confusion regarding their usage in web development. Thus, the Heading Tag is the correct choice as it directly fulfills the function of defining headers in HTML documents.