A top-level domain (TLD) is the last part of a domain name, such as
TLDs for web development
Many developers use a TLD for local domains for development.
.dev is often used for this, but was registered in 2015 by Google . In 2017, Google enabled HSTS for all
.dev domains  meaning it will not allow to work over HTTP any more.
- 2017-10-22 : macOS 10.13 High Sierra Apache Setup: MySQL, APC & More... (archived) - a tutorial on using Apache Virtual Hosts and Dnsmasq on macOS to set-up a wildcard TLD.
Database of available TLDs
The IANA manages the authoritative list of TLDs on their website: the Root Zone Database. Every domain has its own Delegation Record that is linked to and can be checked. If you would like to know who owns
.cat you can check its Delegation Record.