A term peculiar to mortgage and contract law to refer to the borrower of money secured by a mortgage.
The mortgagor pledges property (usually real property) as security on the loan.
The counterpart to the mortgagor is the person lending on the basis of a mortgage; the mortgagee (usually a bank but often a private individual).
A mortgagor usually has a right of redemption which requires the lender or mortgagee to defeat before he can take possession of the real property and sell it to pay off the secured loan. That process is known as foreclosure.