The Free On-line Dictionary of Computing (30 December 2018):

top-level domain
TLD

    The last and most significant component of an
   Internet fully qualified domain name, the part after the
   last ".".  For example, host wombat.doc.ic.ac.uk is in
   top-level domain "uk" (for United Kingdom).

   Every other country has its own top-level domain, including
   ".us" for the U.S.A.  Within the .us domain, there are
   subdomains for the fifty states, each generally with a name
   identical to the state's postal abbreviation.  These are
   rarely used however.  Within the .uk domain, there is a .ac.uk
   subdomain for academic sites and a .co.uk domain for
   commercial ones.  Other top-level domains may be divided up in
   similar ways.

   In the US and some other countries, the following top-level
   domains are used much more widely than the country code:

   	.com - commercial bodies
   	.edu - educational institutions
   	.gov - U. S. government
   	.mil - U. S. armed services
   	.net - network operators
   	.org - other organisations

   Since the rapid commercialisation of the Internet in the 1990s
   the ".com" domain has become particularly heavily populated
   with every company trying to register its company name as a
   subdomain of .com, e.g. "netscape.com" so as to make it easy
   for customers to guess or remember the URL of the comany's
   home page.

   United Nations entities use the domain names of the countries
   where they are located.  The UN headquarters facility in New
   York City, for example, is un.org.

   Several new top-level domains are about to be added (Oct
   1997):
   	.nom   - individual people
   	.rec   - recreational organisations
   	.firm  - businesses such as law, accounting, engineering
   	.store - commercial retail companies
   	.ent   - entertainment facilities and organisations

   (1997-10-08)