It stands for specific collateral rate; a repo rate that corresponds to specific collateral such as Treasury securities that are on the run or have recently been issued. This rate is usually lower than the repo rate available on less actively traded Treasury securities. The spread between the general collateral rate and the special repo rate is usually referred to as the repo spread or the specialness repo. This spread can be viewed as a repo dividend (it goes to the holder of collateral, i.e., a dealer or the investor holding the underlying instrument that is on special).
This rate is also known as a special repo rate.
