I think "not present in the routing table" is a good working definiion of "unused".
Not according to RFC 2050.
If a market does happen, it will be interesting to see how much of that "unreclaimable" address space appears on that market.
Note that most of that address space which does not appear in the global routing table is actually in use on internetworks that, as a matter of policy, do not exchange packets with the public Internet. Since these are internetworks, use of RFC 1918 addressing is not possible, and since the end users of these internetworks are also connected to the public Internet, any reuse of these addresses on the public Internet could disrupt the operation of the hidden internetworks. No matter what direction these IPv4 discussions take, it just reinforces the necessity of getting IPv6 up and running as soon as possible. --Michael Dillon