A "header" is a section of text that appears at the top of a document or webpage. It typically contains important information such as the title, author, date, or logo. In web design, headers help organize content and improve navigation, making it easier for users to find what they need. In programming, particularly in languages like HTML, a header can refer to specific tags, such as <h1> to <h6>, which define headings of different levels. These tags help structure content hierarchically, enhancing readability and search engine optimization (SEO) by indicating the importance of various sections.