By pure coincidence I stumbled upon a dictionary of "unusual words" and yep, found it: "Emphyteusis".
There are various definitions, depending on which legal system it was implemented under.
"In the Roman and civil law. A contract by which a landed estate was leased to a tenant, either in perpetuity or for a long term of years, upon the reservation of an annual rent or canon, and upou the condition that the lessee should improve theproperty, by building, cultivating, or otherwise, and with a right in the lessee to alien the estate at pleasure or pass it to his heirs by descent, and free from any revocation.re-entry, or claim of forfeiture on the part of the grantor, except for non-payment of the rent."
"An emphyteutic lease is a type of real estate contract specifying that the lessee must improve the property with construction. The term is commonly used in Quebec. These sorts of leases are usually associated with government properties."
The key difference between an emphyteusis lease and a normal one, seems to be that there is a condition that stipulates that the leesee must put the land to good use, improve it somehow, either cultivate it or develop it.