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): Two letter domains established for countries or territories. With some historical exceptions, the code for any territory is the same as its two-letter ISO 3166 code.
• generic top-level domains (gTLD): Top-level domains with three or more characters
• unsponsored top-level domains: domains that operate directly under policies established by ICANN processes for the global Internet community.
• 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. Use is based on community theme concepts.
• infrastructure top-level domain: This group consists of one domain, the Address and Routing Parameter Area (ARPA). It is managed by IANA on behalf of the Internet Engineering Task Force for various purposes specified in Request for Comments.

The domain arpa was the first Internet top-level domain. It was intended to be used only temporarily, aiding in the transition of traditional ARPANET host names to the domain name system. However, after it had been used for reverse DNS lookup, it was found impractical to retire it, and is used today exclusively for Internet infrastructure purposes such as in-addr.arpa for IPv4 and ip6.arpa for IPv6 reverse DNS resolution, uri.arpa and urn.arpa for the Dynamic Delegation Discovery System, and e164.arpa for telephone number mapping based on NAPTR DNS records. For historical reasons, arpa is sometimes considered to be a generic top-level domain.

In the late 1980s InterNIC created the nato domain for use by NATO. NATO considered none of the then existing TLDs as adequately reflecting their status as an international organization. Soon after this addition, however, InterNIC also created the int TLD for the use by international organizations in general, and persuaded NATO to use the second level domain nato.int instead. The nato TLD, no longer used, was finally removed in July 1996.

Other historical TLDs are cs for Czechoslovakia (now cz for Czech Republic and sk for Slovak Republic), dd for East Germany (using de after reunification of Germany), and zr for Zaire (now cd for Democratic Republic of the Congo). In contrast to these, the TLD su has remained active despite the demise of the Soviet Union that it represents.

About the time that ICANN discussed and finally introduced[8] aero, biz, coop, info, museum, name, and pro TLDs, site owners argued that a similar TLD should be made available for adult and pornographic websites to settle the dispute of obscene content on the Internet and the responsibility of service providers under the Communications Decency Act of 1996. Several options were proposed including xxx, sex and adult, but ICANN has not created any.

An older proposalconsisted of seven new gTLDs arts, firm, info, nom, rec, shop, and web. Later biz, info, museum, and name covered most of these old proposals.

During the 32nd International Public ICANN Meeting in Paris in 2008,ICANN started a new process of TLD naming policy to take a "significant step forward on the introduction of new generic top-level domains." This program envisions the availability of many new or already proposed domains, as well a new application and implementation process. Observers believed that the new rules could result in hundreds of new gTLDs to be registered. Proposed TLDs include music, berlin and nyc.

ICANN's slow progress in creating new generic top-level domains, and the high application costs associated with TLDs, contributed to the creation of alternate DNS roots with different sets of top-level domains. Such domains may be accessed by configuration of a computer with alternate or additional (forwarder) DNS servers or plugin modules for web browsers. Browser plugins detect alternate root domain requests and access an alternate domain name server for such requests.

Several networks, such as BITNET, CSNET or other UUCP networks, existed that were in wide-spread use among computer professionals and academic users, that were incompatible with the Internet and exchanged e-mail with the Internet via special e-mail gateways. For relaying purposes on the gateways, messages associated with these networks were labeled with suffixes such as bitnet, oz, csnet, or uucp, but these domains did not exist as top-level domains in the public Domain Name System of the Internet.

Most of these networks have long since ceased to exist, and although UUCP still gets significant use in parts of the world where Internet infrastructure has not yet become well-established, it subsequently transitioned to using Internet domain names, so pseudo-domains now largely survive as historical relics. One notable exception is the 2007 emergence of SWIFTNet Mail, which uses the swift pseudo-domain.

The top-level pseudo domain local is required by the Zeroconf protocol. It is also used by many organizations internally, which may become a problem for those users as Zeroconf becomes more popular. Both site and internal have been suggested for private usage, but no consensus has emerged[citation needed].

The anonymity network Tor has a top-level pseudo-domain onion, which can only be reached with a Tor client because it uses the Tor-protocol (onion routing) to reach the hidden service in order to protect the anonymity of users.