RFC 1034 Domain Concepts and Facilities November 1987 [Page 8]

"A domain is identified by a domain name, and consists of that part of the domain name space that is at or below the domain name which specifies the domain. 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 " "."

   1 cdb.cr.yp.to - regular DNS:
   124 bytes, 1+2+0+0 records, response, noerror
   query: 1 cdb.cr.yp.to
   answer: cdb.cr.yp.to 30 CNAME pq1jbw2qzb2201xj6pyx177b8frqltf7t4wdpp32fhk0w3h70uytq5020w020l0.yp.to
   answer: pq1jbw2qzb2201xj6pyx177b8frqltf7t4wdpp32fhk0w3h70uytq5020w020l0.yp.to 30 A 131.193.32.109

In the terminology of RFC1034, cdb.cr.yp.to, a CNAME, can be described as a subdomain of cr.yp.to and yp.to

(NB. The pq1 portion is not a public key, it is a hash of a server's long-term public key)