One general term, though it's kind of unsatisfying, is "economic land," and it can refer to things like electromagnetic spectrum too. Any finite resource that that is a factor of production and can not be expanded.

https://en.wikipedia.org/wiki/Land_(economics)

For namespaces, there is extra value for particular names that come not from any work the owner of the name performed, but rather that comes from the productive activity of others. If I squat "olives.com" in 1994 and wait for the web to get big, and then sell the domain in 2000 to a hottest new direct to consumer olive startup, then any profit from that should really be taxed away. All that "value" came from the work of others that made the web into a valuable place to own some "land."

Maybe "inelastic resource tax" or "squatting tax"?