mortgage
(a) The term “mortgage” means a first mortgage on real estate, in fee simple, or on a leasehold (1) under a lease for not less than ninety-nine years which is renewable; or (2) under a lease for a period of not less than fifty years to run from the date the mortgage was executed; and the term “first mortgage” means such classes of first liens as are commonly given to secure advances on, or the unpaid purchase price of, real estate, under the laws of the State in which the real estate is located, together with the credit instruments, if any, secured thereby.