Explanation:
Milestones are typically major accomplishments of the project and mark the completion of major deliverables or some other key event in the project. For example, approval and sign-off on project deliverables might be considered milestones. Other examples might be the completion of a prototype, functional testing, contract approval, and so on.
A milestone is typically denoted on a project schedule as an event that is achieved once all the deliverables associated with that milestone are completed and it has a duration of zero.