In the Domain Name System (DNS) hierarchy, a subdomain is a domain that is a part of a main domain. It is a domain that is part of a larger domain; the only domain that is not also a subdomain is the root domain. For example ‘shop.apple.com‘ is a sub domain of ‘apple.com‘ domain, which in turn is a sub domain of the ‘.com‘ top level domain (TLD). Alternatively just the word ‘shop‘ is also referred to as the sub domain in this instance.