Which of the following describes a subdomain?

A subdomain is best described as a segment of the main domain, often used to organize content. Subdomains are created to break down a website into more specific parts or categories without needing to register a whole new domain name. For example, if the main domain is "example.com," a subdomain could be "blog.example.com" or "store.example.com." This structure helps in organizing content under the main domain, allowing for easier navigation and better site management.

The other options do not accurately define a subdomain. A unique URL for a specific page describes a specific webpage rather than the broader concept of a subdomain. An area dedicated solely for e-commerce suggests a specific function or purpose rather than the organizational aspect that a subdomain provides. Lastly, a secondary website without any relation to the main domain implies a disconnect that is not characteristic of how subdomains function, as they are inherently connected to the main domain.