A milestone is a scheduled event signifying the completion of a major deliverable or a set of related deliverables. A milestone has zero duration and no effort -- there is no work associated with a milestone. It is a flag in the workplan to signify some other work has completed.
Usually a milestone is used as a project checkpoint to validate how the project is progressing and revalidate work. Milestones are also used as high-level snapshots for management to validate the progress of the project. In many cases there is a decision to be made at a milestone.
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