A hostname is the name part of a DNS record as opposed to the data content such as an IP address.

For example; ns1.domainmonster.com has the following A-record: Name:ns1, Type:a, Data:IP

__________________
Domainmonster.com Customer Support Team.
Domain Name Registration Information & Resources.

I've never really understood the difference between a hostname and a subdomain..

I think hostname comes from the fact that machines are given a "name" for the "host" machine on the network.

So on a network "machine2" on the local domain "mynetwork.net" becomes machine2.mynetwork.net.

As the internet developed, and hosts/servers have become more virtual, the term subdomain was used because it didn't necessarily mean that it was a different server or "host".