Sometimes your stakeholders will write the project scope. At the outset of the project, they might only have a vague idea of what will be required of the project. As the project is planned, you might discover that important facets were left out. When you bring this to the attention of the stakeholders, they think of further requirements for the project scope. This goes back and forth a few times. When are these additions scope creep and when are they scope discovery? Here is a general rule of thumb:
If additions to the project scope refine and make the project scope more clear and less vague, most likely you have a case of scope discovery.
If additions to the project scope make the project scope less clear, more vague and more ambiguous, most likely, you have a case of scope creep.
An enlarging scope is a natural part of the project process. You should not panic anytime your project's requirements enlarge. Instead, ask yourself whether you have a true case of scope creep. For example, if you are working on a software project where the stakeholders defined the project scope as being, "Create a software program that allows users to track time" and the following stipulation arises during the project planning process, "The software should track time any time a file is opened on a computer." This stipulation further defines the project scope, making it a case of scope discovery. If, on the other hand, when you submit this to the stakeholders they say, "Wouldn't it be nice if the program also tracked everything someone ate in a day, and their calories expended," then you may have a case of scope creep.
A final thought: scope creep is often a result of poor communication at the outset of the project. Before beginning the project completion phase, be sure that you have the scope pinned down as close as possible to the intentions of the stakeholders. This is one of the best ways to avoid the negative aspects of scope creep.