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A third-level domain is the part of a domain name added to the left of the second-level domain. For example, in blog.nic.ua, blog is the third-level domain, nic is the second-level domain (SLD), and .ua is the top-level domain (TLD).
In simple terms, a third-level domain is often used as a subdomain for separate website sections or services, such as shop.example.com, mail.example.com, or support.example.com. It helps organize content, services, and business directions within one main domain.
It is the part of the address placed before the main domain. For example, in blog.site.com, the word blog is a third-level domain. It is often called a subdomain.
In practice, in most cases they mean the same thing: a third-level domain is usually called a subdomain. For example, blog.example.com, mail.example.com, and shop.example.com are subdomains (third-level domains).
It is used to separate website sections and services: blog, store, email, support, user account, and more. This is convenient for project structure, administration, and clear navigation for users.