It stands for conditional prepayment yield; a measure of prepayment speed in asset backed securities (ABS) and other specific structures/ instruments (CMBS). This measure relates to the conditional prepayment rate (CPR)- i.e., the annualized percentage of an existing mortgage pool that is expected to be prepaid in a year.

A 100% conditional prepayment yield (CPY) is the equivalent of 0% CPR up to the end of the yield maintenance (YM) period and 100% CPR later on. Yield maintenance represents a prepayment premium that allows investors (holders of the securities) to attain the same level of yield assuming that the borrower made all interest payments on a timely basis.