An S-Curve is a sigmoid function, that is a mathematical process or function that results in a S shaped curve also called a Sigmoid Curve. The S-Curve is used in project management as a means of representing the various expenditures of resources over the projected time of the project or as a means of charting the real-time expenditure of resources. This is important to project management in that it can be used to monitor the project as is progresses and compare it to the projected S-Curve to determine whether or the project is being completed within the time and budget limitations. These resources might be the cumulative cost of the project, the number of man hours required at any given stage in the project, the expenditure of raw materials for construction or assembly, etc.
The term S-Curve can also be used to indicate an S shaped chart resulting from a cumulative likelihood distribution. In this function, an S-Curve is a tool of quantitative risk analysis which project management would use to determine the possible dangers of any given course of action.
This term is defined in the 3rd and the 4th edition of the PMBOK.