DNS/TLDについて、ここに記述してください。

https://datatracker.ietf.org/doc/html/rfc8499


Domain name:  An ordered list of one or more labels.

      Note that this is a definition independent of the DNS RFCs
      ([RFC1034] and [RFC1035]), and the definition here also applies to
      systems other than the DNS.  

   [RFC1034] defines the "domain name space" using mathematical trees and 
   their nodes in graph theory, and that definition has the same practical result
   as the definition here.  

      Any path of a directed acyclic graph can be
      represented by a domain name consisting of the labels of its
      nodes, ordered by decreasing distance from the root(s) (which is
      the normal convention within the DNS, including this document).  A
      domain name whose last label identifies a root of the graph is
      fully qualified; other domain names whose labels form a strict
      prefix of a fully-qualified domain name are relative to its first
      omitted node.

      Also note that different IETF and non-IETF documents have used the
      term "domain name" in many different ways.  It is common for
      earlier documents to use "domain name" to mean "names that match
      the syntax in [RFC1035]", but possibly with additional rules such
      as "and are, or will be, resolvable in the global DNS" or "but
      only using the presentation format".


Top-Level Domain (TLD):  A Top-Level Domain is a zone that is one
      layer below the root, such as "com" or "jp".  There is nothing
      special, from the point of view of the DNS, about TLDs.  Most of
      them are also delegation-centric zones (defined in Section 7), and
      there are significant policy issues around their operation.  TLDs
      are often divided into sub-groups such as Country Code Top-Level
      Domains (ccTLDs), Generic Top-Level Domains (gTLDs), and others;
      the division is a matter of policy and beyond the scope of this
      document.

Subdomain:  "A domain is a subdomain of another domain if it is
      contained within that domain.  This relationship can be tested by
      seeing if the subdomain's name ends with the containing domain's
      name."  (Quoted from [RFC1034], Section 3.1) For example, in the
      host name "nnn.mmm.example.com", both "mmm.example.com" and
      "nnn.mmm.example.com" are subdomains of "example.com".  Note that
      the comparisons here are done on whole labels; that is,
      "ooo.example.com" is not a subdomain of "oo.example.com".