CDO-squared is a financial instrument that was popular in the mid-2000s. It is essentially a collateralized debt obligation (CDO) that is backed by other CDOs. However, this complex financial instrument has earned a poor reputation due to its role in the 2008 financial crisis. Therefore, there are a number of synonyms that have become associated with this term. Some people refer to it as a "synthetic CDO," while others call it a "multi-layered CDO." Additionally, some people simply refer to it as a "CDO2" or "CDO squared." Whatever the term used, this type of financial instrument is generally considered to be high-risk and often associated with financial instability.