Scope creep is the gradual increase of scope without formal change control. It generally occurs because the change seems too small to matter or isn't even recognized as an addition to the approved requirements.
Scope creep is a major cause of project delays. By increasing the complexity of the project it can also create other problems. And since these changes are not being reviewed formally, even if they are helpful where suggested, they may interfere with other parts of the project.
Related: control scope
- Wikipedia on scope creep