A top-level domain (TLD) is one of the domains at the highest level in the hierarchical Domain Name System of the Internet. The top-level domain names are installed in the root zone of the name space. For all domains in lower levels, it is the last part of the domain name, that is, the last label of a fully qualified domain name. For example, in the domain name www.example.com, the top-level domain is com. Responsibility for management of most top-level domains is delegated to specific 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.

As of 2015, IANA distinguishes the following groups of top-level domains:

• infrastructure top-level domain (ARPA): This group consists of one domain, the Address and Routing Parameter Area. It is managed by IANA on behalf of the Internet Engineering Task Force for various purposes specified in the Request for Comments publications.
• generic top-level domains (gTLD): Top-level domains with three or more characters
• restricted generic top-level domains (grTLD): These domains are managed under official ICANN accredited registrars.
• 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; these domains are managed under official ICANN accredited registrars.
• 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 3166code.
  • internationalized country code top-level domains (IDN ccTLD): ccTLDs in non-Latin character sets (e.g., Arabic, Cyrillic, Hebrew, or Chinese).
• test top-level domains (tTLD): These domains were installed under .test for testing purposes in the IDN development process; these domains are not present in the root zone.

Countries are designated in the Domain Name System by their two-letter ISO country code;there are exceptions, however (e.g., .uk). This group of domains is therefore commonly known as country-code top-level domains (ccTLD). Since 2009, countries with non–Latin-based scripts may apply for internationalized country code top-level domain names, which are displayed in end-user applications in their language-native script or alphabet, but use a Punycode-translated ASCII domain name in the Domain Name System.

Generic top-level domains (formerly Categories) initially consisted of gov, edu, com, mil, org, and net. More generic TLDs have been added, such as info.

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