Scope Creep
Scope creep within a project happens when the project takes on tasks, activities, outcomes, or deliverables outside the defined scope or beyond the project plan. Most simply put, scope creep is expanding the project beyond its original goals. Scope creep could include a new data source, adding questions to a survey, more reports than originally requested, or evaluating a piece of the project that wasn't initially included. Watching for scope creep is important as any changes to the project plan can affect budget, timelines, and reports. If adding aspects that weren't included in the project plan are important, these need to be agreed upon and changes made to the plan as needed.
See also: scope, Scope Creep: When to Indulge it, and When to Avoid it