[gst]

If you define "unused" domain as "doesn't have a working website associated to it" then this definition is (imo) wrong. The internet is more than just websites. I have several domains that don't have any websites associated with them, but I still use them for things such as email, hostnames, etc.

[pmontra]

That's a legitimate use and that's another reason for it is difficult to detect if a domain is used.