ETYM French mort-gage; mort dead (Latin mortuus) + gage pledge. Related to Mortal, and Gage.
A conditional conveyance of property as security for the repayment of a loan.
The pledging of real property—usually a building—as security for repayment of a loan. The loan and contracted interest is repaid over a contracted period of years, at which time ownership of the property reverts to the titleholder.

To put up as security or collateral.