A subdomain is a subdivision of a main domain name, used to organize and navigate to different sections of a website. It appears before the main domain in a URL, separated by a dot (e.g., blog.example.com). Subdomains allow website owners to create distinct areas for specific content or functions such as support, shop, or blog without needing to register a new domain. They are commonly used for testing environments, regional sites, or specialized services within the same overall web presence.