The difference between yields on Treasury securities and non-Treasury securities with identical characteristics except for quality rating.

This is also sometimes known as the credit spread, although credit spread has other definitions as well. An example of quality spread would be the difference between yields on Treasuries and those on single A-rated industrial bonds. In this situation a company is forced to offer a higher return on their bonds because their credit is not as good as the government's credit. From the perspective of the investor, profit will be higher if the spread narrows.