What is it called when requirements are added after the initial project has been defined and approved, affecting costs or timelines?

The phenomenon described in the question is known as scope creep. This term refers to the gradual expansion of project requirements after the initial definition and approval stages. Scope creep can occur when stakeholders request additional features, changes, or enhancements to a project after its scope has been established. Consequently, these unplanned additions often lead to increases in costs and delays in timelines, as teams must accommodate the new requirements alongside existing commitments.

In project management, controlling scope creep is crucial for maintaining the original project goals, cost estimates, and deadlines. By establishing a clear scope definition and a structured change management process, project managers can better manage stakeholder expectations and mitigate the risks associated with unwarranted expansions of the project scope.