Scope creep is the gradual, usually uncontracted expansion of a project's scope during delivery.
Causes. Imprecise scope of work, no change control, undefined acceptance criteria.
Effect by pricing model. Under T&M, immediate additional cost; under fixed price, change requests or reduced quality.
Controls. Exclusions in the scope of work, a change request procedure, defined acceptance criteria, a not-to-exceed cap.
Not the same as change. A documented extension is a change request; scope creep is the undocumented version.