Debt Security

You could make an argument for "debt security" as an oxymoron, along the lines of "jumbo shrimp" or "happily married." After all, mounting debt makes finances less secure.

However, that's not what the kind of security at play here. A "security" in this context is something tradable. A share of stock is an equity security. A debt security represents a tradable form of debt. Easy examples: Bonds, notes, and commercial paper.

So instead of negotiating a loan from a particular bank, a company will issue debt instruments (like bonds) and sell those on the open market. Investors can then trade them among themselves. It's a way to get a larger number of people involved in the money-raising process.