The gradual growth of a project beyond what was agreed, through small additions that each look harmless. No single request causes the overrun; their sum does.
Scope creep is the most common reason projects are late and over budget. It rarely comes from bad intent — it comes from an unclear baseline and from changes that are approved individually but never re-costed together.
We fix a written scope at the start and treat every addition as a visible trade: it either replaces something, moves the date, or goes to the next release. You always see the running cost of changes, not a surprise at the end.