H3, or HTML heading 3, is a tag used in web development to define the third level of headings in a document. It helps organize content hierarchically, making it easier for readers to navigate and understand the structure of a webpage. H3 tags are typically used for subheadings under H2 tags, providing a clear outline of topics and subtopics. In addition to improving readability, H3 tags also play a role in search engine optimization (SEO). Search engines use headings to understand the content of a page better, which can influence how well a site ranks in search results. Proper use of H3 tags can enhance both user experience and visibility online.