MoSCoW is a technique used in management, business analysis, and software development to reach a common understanding with stakeholders on the importance they place on the delivery of each requirement - also known as MoSCoW prioritization or MoSCoW analysis.
According to A Guide to the Business Analysis Body of Knowledge, version 2.0, section 184.108.40.206, the MoSCoW categories are as follows:
M - MUST: Describes a requirement that must be satisfied in the final solution for the solution to be considered a success.
S - SHOULD: Represents a high-priority item that should be included in the solution if it is possible. This is often a critical requirement but one which can be satisfied in other ways if strictly necessary.
C - COULD: Describes a requirement which is considered desirable but not necessary. This will be included if time and resources permit.
W - WON'T: Represents a requirement that stakeholders have agreed will not be implemented in a given release, but may be considered for the future. (note: occasionally the word "Would" is substituted for "Won't" to give a clearer understanding of this choice)