The MoSCoW rules are a method for prioritizing requirements used quite widely in the United Kingdom especially by followers of the Dynamic System Development Method (DSDM). In The Dynamic System Development Method, Jennifer Stapleton introduces the MoSCoW rules thus:

You will not find the MoSCoW rules in the DSDM Manual, but they have been adopted by many organizations using DSDM as an excellent way of managing the relative priorities of requirements in a RAD project. They are the brainchild of Dai Clegg of Oracle UK, who was one of the early participants in the DSDM Consortium.