A top-level domain or domain name (TLD) is the highest level of domain names in the root zone of the Domain Name System of the Internet. For all domains in lower levels, it is the last part of the domain name, that is, the label that follows the last dot of a fully qualified domain name. For example, in the domain name www.example.com, the top-level domain is com (or COM, as domain names are not case-sensitive). Management of most top-level domains is delegated to responsible organizations by the Internet Corporation for Assigned Names and Numbers (ICANN), which operates the Internet Assigned Numbers Authority (IANA) and is in charge of maintaining the DNS root zone.

Originally, the top-level domain space was organized into three main groups, Countries, Categories, and Multiorganizations. An additional temporary group consisted only of the initial DNS domain, arpa, intended for transitional purposes toward the stabilization of the domain name system.

Countries are designated in the domain name system by their English two-letter ISO country code. This group of domains is therefore commonly known as country-code top-level domains (ccTLD).

The Categories group has become known as the generic top-level domains. Initially this group consisted of GOV, EDU, COM, MIL, ORG, and NET.

In the growth of the Internet, it became desirable to create additional generic top-level domains. Some of the initial domains' purposes were also generalized, modified, or assigned for maintanance to special organizations affiliated with the intended purpose.

As a result, IANA today distinguishes the following groups of top-level domains:

• country-code top-level domains (ccTLD): Used by a country or a dependent territory. It is two letters long, for example .us for the United States. With some historical exceptions, the code for any territory is the same as its two-letter ISO 3166 code.
• generic top-level domains (gTLD): Generic domains are essentially open for registration to anyone in the world.
  • generic-restricted top-level domains: Similar to general generic group, except eligibility is supposed to be restricted and ascertained more stringently.
  • sponsored top-level domains (sTLD): These domains are proposed and sponsored by private agencies or organizations that establish and enforce rules restricting the eligibility to use the TLD. Often they are also grouped with the generic top-level domains.
• infrastructure top-level domain: This group consists of one TLD, the Address and Routing Parameter Area (ARPA). It is managed by IANA on behalf of the IETF for various purposes specified in RFCs.

In addition, a group of internationalized domain name (IDN) top-level domains has been installed under test for testing purposes.

The authoritative list of currently existing TLDs in the root zone is published at the IANA website at http://www.iana.org/domains/root/db/ and a Wikipedia list exists.