Definition of "subdomain"

subdomain

noun
plural subdomains
1. A domain or sphere that is part of a larger domain or sphere.
  Quotations
  As a technical subdomain of biology ecological investigations have evolved from what has been traditionally called natural history.
  1971, Jacob Robert Kantor, The logic of modern science
2. (computing, Internet) A domain name that has been prefaced with additional parts separated with periods.
  Quotations
  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 " ".
  1987 November, P. Mockapetris, Domain Concepts and Facilities, RFC1034, also known as STD0013
  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.
  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
3. (computing, Internet) An additional part prefaced to a domain name.