A loan where the borrower grants the lender a security interest over its property to mitigate the credit risk.

A home loan is a great example. The Lender gives you money; you grant the lender a mortgage over the title to your house.

Not all forms of collateral are security interests of course: title transfer collateral arrangements, for very good example. So something can resemble a secured loan without actually being one.

A repo resembles a secured loan, but isn’t either secured or, for that matter, a loan. Likewise, a stock loan is not a loan and nor is it secured, unless you are using the new pledge GMSLA form, in which case it is secured, but still is not a loan.