Scope Creep

'Scope Creep' refers to the common practice of customer's changing their mind as to what their requirements are or of  events outside a project changing what the deliverables will need to do or the required performance.

Care must be taken to differentiate between legitimate scope creep (the environment has changed so the deliverable must change to still deliver the required benefits) and scope creep caused by the customer wanting 'would be nice' features or additional unnecessary functionality when they have paid only for vital functionality and features.