A zone is the combination of the
top level domain (currently, these include:
.com,
.edu,
.org,
.net,
.gov,
.int,
.mil,
.arpa, and the
country codes) and the secondary domain. Some examples of zones would be:
slashdot.org,
colorado.edu, and
ibm.com.
Many refer to this as the "domain name," although this is incorrect. A domain name includes both the zone, and any additional specifying information (further subdomains or a hostname). For my above examples, cs.colorado.edu, ftp.slashdot.org, and www.ibm.com would all be "domain names."