The HTML5 element used for a sidebar or related content is <aside>.
The <aside> element represents content that is indirectly related to the main content around it. Typical examples include sidebars, pull quotes, author biographies, related links, advertising, and supplementary notes. Its meaning is structural and semantic; CSS determines whether it appears on the side, below the article, or elsewhere on the page.
A common mix-up is <nav>, which is specifically intended for major navigation links. <article> represents a self-contained composition that could stand on its own, such as a news story or forum post, while <section> groups content by theme, usually with a heading. An <aside> can appear inside an <article> when the supplementary content relates only to that article, or outside it when it supports the page more broadly.
HTML5 introduced these semantic elements to make document structure clearer to browsers, search engines, assistive technologies, and developers than generic <div> elements alone.