Quality spread

Quality spread is, the spread between Treasury securities and non-Treasury securities that are identical in all respects except for quality rating. For instance, the difference between yields on Treasuries and those on single A-rated industrial bonds. It is also called credit spread. 

In an interest rate swap quality spread differential is the difference between the interest rates of debt obligations offered by two parties of different creditworthiness that engage in the swap. A swap transaction is considered beneficial to both parties only when the QSD is positive.