subdomain

noun3

1. •
   A domain or sphere that is part of a larger domain or sphere.

   • As a technical subdomain of biology ecological investigations have evolved from what has been traditionally called natural history. — 1971
2. •
   (Internet) A domain name that has been prefaced with additional parts separated with periods.

   • 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. •
   (Internet) An additional part prefaced to a domain name.

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