Final answer:
A Milestone is the point in the project plan marking the completion of vital tasks that significantly impact the progress of the project.
Step-by-step explanation:
The specific point in the project plan when a task and its action steps are complete and have a noticeable impact on the progress of the project plan is known as a Milestone. Milestones are significant events or achievements that mark the completion of a major phase of work. They help to measure progress toward the achievement of the project's goals. Determining when a milestone is accomplished involves establishing clear, measurable criteria.