What is a Milestone?
A milestone is a significant point or event in the project — a phase gate passed, a major deliverable accepted, a permit received. It has zero duration: it marks a moment, not work. That zero-duration detail is a favorite exam fact.
Milestones earn their keep in communication. Executives don't want your 400-line schedule; they want the milestone chart: ten dates that tell the story. Miss one and everyone instantly understands the project's health — which is exactly why you choose them carefully.
Worked example
A data-center build tracks six milestones: design approved, permits issued, shell complete, power live, racks installed, go-live. "Power live" slipping three weeks tells the steering committee more in one glance than any status report — and triggers the schedule-compression conversation early.