These are definitions of terms used by the Internet Domain Survey. Not everyone uses these terms with the same meanings.
A domain name is any name representing any record that exists within the Domain Name System. ie. com, example.com, www.example.com
A domain is a domain name that has name server (NS) records associated with it. In other words, there may be subdomains or hosts under it. ie. com, example.com
A domain server is a computer system that holds all the records associated with a particular domain, and answers queries about those names. Often called a domain name server or just a name server.
A top-level domain name, or TLD, is either an ISO country code or one of the generic domains (com/org/net/etc). It should be noted that there is not necessarily any correlation between a country code and where a host is actually located. Domains named after ISO country codes are often referred to as “ccTLDs”, and generic domains are often referred to as “gTLDs”.
A duplicate host is a single host name that maps to multiple IP addresses. When a host like this is found in the survey, it is only counted once.
A host is a domain name that has an IP address (A) record associated with it. This would be any computer system connected to the Internet (via full or part-time, direct or dialup connections). ie. example.com, www.example.com
host name (or firstname)
A host name is the first part (before the first dot) of a hosts’ domain name. ie. www
network numbers (class a/b/c)
A network number is the first part of an IP address that identifies what network the hosts numbered in the rest of the address are connected to. The Class A/B/C system is no longer used and the data is presented that way only for historical reasons. It is no longer possible to tell how many network numbers are in use by looking at IP addresses, since the netmask can not be determined from the number. Also, note that domains do not map directly to particular network numbers and no correlation between them can be inferred.
A zone transfer is the process of downloading all the records associated with part of a domain from a domain server.