It's also known as the public suffix.
Some examples of public suffixes are:
in the domain name www.example.com, the top-level domain is com.
country code TLC ccTLDs (Country-Code Domain Names). Some countries or geographical regions, such as .us (United States) and .me (Montenegro), sponsor country-code top-level domain names (ccTLDs). These two-letter domain name extensions have special rules and requirements. More … ccTld on godaddy)
Reserved top-level domains
RFC 6761 reserves the following four top-level domain names to avoid confusion and conflict. Any such reserved usage of those TLDs should not occur in production networks that utilize the global domain name system:
- example: reserved for use in examples
- invalid: reserved for use in obviously invalid domain names
- localhost: reserved to avoid conflict with the traditional use of localhost as a hostname
- test: reserved for use in tests