Definition
A collateralized debt obligation, or CDO, is a financial instrument created by pooling a large set of debt assets — mortgages, loans, bonds — and then issuing new securities backed by the cash flows from that pool. The new securities are divided into layers called tranches.
Each tranche carries a different priority claim on the pool's payments and therefore a different risk-return profile, allowing investors to buy the slice of risk that suits them.
Why it matters
How it works
Payments from the underlying loans flow first to the senior tranche, then to mezzanine tranches, and last to the equity tranche. Senior holders are paid before losses reach them, so their slice is rated safe; junior holders absorb the first losses in exchange for higher yield.
The model assumes losses in the pool are largely independent and predictable. In 2008 that assumption failed: housing-linked loans defaulted together, losses cascaded through tranches once thought secure, and complexity meant few could value the products. The episode is a standard caution about financial innovation outrunning understanding of risk.