The opposite of secured debt/loan is unsecured debt, which is not connected to any specific piece of property. Instead, the creditor may satisfy the debt only against the borrower, rather than the borrower's collateral and the borrower.
The opposite of secured debt/loan is unsecured debt, which is not tied to any specific asset. Instead, the creditor can only collect on the debt from the borrower directly, rather than relying on collateral or property.