https://en.wikipedia.org/wiki/Subdomain

In the Domain Name System (DNS) hierarchy, a subdomain is a domain that is a part of another (main) domain

 

The Domain Name System (DNS) has a tree structure or hierarchy, with each non-RR (resource record) node on the tree being a domain name. A subdomain is a domain that is part of a larger domain; the only domain that is not also a subdomain is the root domain.^[1]

For example, west.example.com and east.example.com are subdomains of the example.com domain, which in turn is a subdomain of the com top-level domain (TLD).

A "subdomain" expresses relative dependence, not absolute dependence: for example, wikipedia.org comprises a subdomain of the org domain, and en.wikipedia.org comprises a subdomain of the domain wikipedia.org.

Each label may contain from 1 to 63 octets. The empty label is reserved for the root node and when fully qualified is expressed as the empty label terminated by a dot. The full domain name may not exceed a total length of 253 ASCII characters in its textual representation.^[2] Thus, when using a single character per label, the limit is 127 levels: 127 characters plus 126 dots have a total length of 253. In practice, some domain registries may have shorter limits.

Subdomains in this context are defined by editing the DNS zone file pertaining to the parent domain. However, there is an ongoing debate over the use of the term “subdomain” when referring to names which map to the Address record A (host) and various other types of zone records which may map to any public IP address destination and any type of server. Network Operations teams insist that it is inappropriate to use the term “subdomain” to refer to any mapping other than that provided by zone NS (name server) records and any server-destination other than that.

According to RFC 1034, "a domain is a subdomain of another domain if it is contained within that domain". Based on that definition, a host cannot be a subdomain, only a domain can be a subdomain. A subdomain will also have a separate zone file with a SOA record (Start of Authority). Merely looking at a URL will not tell you if the left most node is a host or a subdomain.