A "header" is a section at the top of a document or webpage that typically contains important information such as the title, author, and date. In web design, headers help organize content and improve navigation by providing links to other sections of the site. In programming, particularly in languages like HTML and CSS, headers are defined using specific tags, such as <h1> for the main title and <h2> for subheadings. This structure not only enhances readability but also aids in search engine optimization (SEO) by helping search engines understand the content hierarchy.