All of those entries are domain names per RFC 1034 and RFC 1035. If they also had the (typically implicit) trailing dot that represents the root they would be fully qualified domain names.
The individual components of a domain name are called labels. These cannot exceed 63 characters in length. A label can include any octet from
If a domain name has an
AAAA) record associated with it (or a
CNAME that ultimately resolves to an
AAAA record) then it is also a hostname. Your three real examples (
www.bbc.co.uk, etc) all qualify.
Special rules apply to hostnames. In particular, every label in a hostname must adhere to the RFC 1123 requirements, i.e. they can only use letter, digit, or hyphens, and hyphens cannot be the first or last character.
Please note that the leftmost label is not a DNS hostname in its own right, although it could be a local (non-DNS) hostname.