In a lending context, security refers to the property that will be pledged to a lender as a guarantee of repayment, which is the same concept more commonly described in full as collateral. A loan backed in this way is described as a secured loan, since the lender has a specific asset it can claim if the borrowerdefaults.
This use of the word is distinct from the unrelated meaning of security as a tradable financial instrument, such as a share or bond, used in investment and capital markets. For the full explanation of how pledged collateral works, what assets are typically used, and how lenders assess and enforce it, see the main Collateral entry.