Milestone
A milestone is a zero-duration event on the schedule — not work, but a point marking that a moment like "acceptance sign-off" has been reached.
A point, not a task
A milestone holds no work, which is why its duration is zero. It pins the gates a project passes — "design approval," "go/no-go meeting" — onto the calendar. On a Gantt chart it is drawn as a diamond, not a bar. Strictly speaking it is a schedule-layer marker, not a structural element of the WBS — though most tools represent it inside the WBS tree as a zero-duration node.
Why it matters
Milestones are the language of reporting. Executives and clients do not ask about task lists; they ask whether gates were passed. "Did we hit the June 20 integration-test-complete milestone?" summarizes hundreds of tasks in one question. Wire predecessors into a milestone and the gate date visibly slips the moment upstream work slips.
Common misconceptions
Registering two weeks of "acceptance support" as a milestone is the classic error — if it has duration, it is a task. Split it: an "acceptance support" task plus an "acceptance sign-off" milestone. Too many milestones is the other failure: when everything is a gate, nothing is.
wbsgantt enforces zero duration on milestones and renders them as diamonds on the Gantt.