As used in this subchapter—

((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 having 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.

((b)) The term “mortgagee” includes the original lender under a mortgage, and his successors and assigns approved by the Secretary; and the term “mortgagor” includes the original borrower under a mortgage and his successors and assigns.

((c)) The term “maturity date” means the date on which the mortgage indebtedness would be extinguished if paid in accordance with periodic payments provided for in the mortgage.

((d)) The term “State” includes the several States, and Puerto Rico, the District of Columbia, Guam, and the Virgin Islands.