Wikipedia
The Z-spread, ZSPRD, Zero-volatility spread or Yield curve spread of a mortgage-backed security (MBS) is the parallel shift or spread over the zero-coupon Treasury yield curve required for discounting a pre-determined cash flow schedule to arrive at its present market price. The Z-spread is also widely used in the credit default swap (CDS) market as a measure of credit spread that is relatively insensitive to the particulars of specific corporate or government bonds.
Since the Z-spread uses the entire yield curve to value the individual cash flows of a bond, it provides a more-realistic valuation than an interpolated yield spread based on a single point of the curve, such as the bond's final maturity date or weighted-average life. However, the Z-spread does not incorporate variability in cash flows, so a fuller valuation of a rate-dependent security often requires the more-realistic (and more-complicated) option-adjusted spread.