What is a Domain Name?

A domain name is a string that identifies a realm of administrative autonomy, authority or control within the Internet. Domains are used in various networking contexts and for application-specific naming and addressing purposes. In general, a domain name identifies a network domain or an Internet Protocol (IP) resource, such as a personal computer used to access the Internet, or a server computer. Domain names are often used to identify services provided through the Internet, such as websites and email services. As of 2017, 330.6 million domain names had been registered.^[1]

Names are formed by the rules and procedures of the Domain Name System (DNS). Any name registered in the DNS is a domain name. Names are organized in subordinate levels (subdomains) of the DNS root domain, which is nameless. The first-level set of domain names are the top-level domains (TLDs), including the generic top-level domains (gTLDs), such as the prominent domains com, info, net, edu, and org, and the country code top-level domains (ccTLDs). Below these top-level domains in the DNS hierarchy are the second-level and third-level domain names that are typically open for reservation by end-users who wish to connect local area networks to the Internet, create other publicly accessible Internet resources or run web sites.

The registration of a second- or third-level name is usually administered by a domain name registrar who sell its services to the public.

A fully qualified domain name (FQDN) is a domain name that is completely specified with all labels in the hierarchy of the DNS, having no parts omitted. Traditionally a FQDN ends in a dot (.) to denote the top of the DNS tree.^[2] Labels in the Domain Name System are case-insensitive, and may therefore be written in any desired capitalization method, but most commonly domain names are written in lowercase in technical contexts.^[3]

Credit https://en.wikipedia.org/wiki/Domain_name