“Lessor” means the person for the time being entitled in reversion expectant on the interest demised, or, where the reversion is encumbered, the person having power to accept a surrender of the lease, or underlease;

“Lessee” and “underlessee” include the persons respectively deriving title under them;

“A perpetually renewable lease or underlease” means a lease or underlease the holder of which is entitled to enforce (whether or not subject to the fulfilment of any condition) the perpetual renewal therof, and includes a lease or underlease for a life or lives or for a term of years, whether determinable with life or lives or not, which is perpetually renewable as aforesaid, but does not include copyhold land held for a life or lives or for years, whether or not determinable with life, where the tenant had before the commencement of this Act a right of perpetual renewal subject or not to the fulfilment of any condition;

“Underlease,” unless the context otherwise requires, includes a subterm created out of a derivative leasehold interest.