Noun

1. a domain name that has been prefaced with additional parts, each part ending in a period
   • 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, 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.

2. Any lower-level part of a domain name.

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

Usage notes