If you parse any domain name, the top-level domain is the last section after the last dot. That’s the highest level within the hierarchy of a domain name. So, for martech.zone, the TLD is .com.
When the web was first launched in the United States, it was fairly easy to remember domain names. .com meant you were on a company’s site, .org meant that you were on a non-profit’s site, .edu meant that you were on a university or school site, .net meant you were on a network, .mil meant you were on a military installation’s site, and .gov meant that you were on a government site. Domain names could be registered for .com, .net, and .org without any restriction but the others were limited to specific purposes.
TLDs are approved and sold by ICANN:
ICANN is a not-for-profit public-benefit corporation with participants from all over the world dedicated to keeping the Internet secure, stable and interoperable. It promotes competition and develops policy on the Internet’s unique identifiers. Through its coordination role of the Internet’s naming system, it does have an important impact on the expansion and evolution of the Internet.
By 2016, 1300 new TLDs were made available for use and, in many cases, for sale to the general public To see an entire list, visit the Root Zone Database, which details all top-level domains, including gTLDs such as .com, and country-code TLDs such as .uk.
Here’s everything you ever wanted to know about TLDs thanks to Hosting Facts.