From sub- +‎ domain.

• Audio (UK):

  (file)

• Rhymes: -eɪn

English Wikipedia has an article on:

subdomain

Wikipedia

subdomain (plural subdomains)

1. A domain or sphere that is part of a larger domain or sphere.
   • 1971, Jacob Robert Kantor, The logic of modern science:

     As a technical subdomain of biology ecological investigations have evolved from what has been traditionally called natural history.

2. (computing, Internet) A domain name that has been prefaced with additional parts separated with periods.
   • 1987 November, P. Mockapetris, Domain Concepts and Facilities, RFC1034, also known as STD0013

     A domain is a subdomain of another domain if it is contained within that domain. This relationship can be tested by seeing if the subdomain's name ends with the containing domain's name. For example, A.B.C.D is a subdomain of B.C.D, C.D, D, and " ".

   • BIND 9 Administrator Reference Manual (9.3.2), Copyright © 2004, 2005 Internet Systems Consortium, Inc. ("ISC"), Copyright © 2000-2003 Internet Software Consortium, https://web.archive.org/web/20060619063455/http://www.bind9.net/manual/bind/9.3.2/Bv9ARM.ch01.html

     Every name in the DNS tree is a domain, even if it is terminal, that is, has no subdomains. Every subdomain is a domain and every domain except the root is also a subdomain.

3. (computing, Internet) An additional part prefaced to a domain name.

   In en.wiktionary.org, en is a subdomain.

The technical community that created the terminology uses the first sense of a "domain name prefaced with additional parts".

• (lower-level part): unqualified hostname

• subsubdomain

• second-level domain
• top-level domain
• hostname