What is the purpose of a milestone?

Question

How is it any different between a goal or a deadline?

Answer 1

Milestones: Terminal Nodes of Major Project Activities

Think of milestones as major markers along your path to project completion. Much like mile markers along the side of the road, a project milestone tells you where you are in your overall project journey. It may be easiest to think of a milestone as a measure of progress or important inflection point along the project's timeline, or as a waypoint on the path to your ultimate destination. Wikipedia says:

Milestones are tools used in project management to mark specific points along a project timeline. These points may signal anchors such as a project start and end date, or a need for external review or input and budget checks.

In many instances, milestones do not impact project duration. Instead, they focus on major progress points that must be reached to achieve success.

However, different frameworks and implementations can (and often do) use milestones in slightly different ways. Various projects might treat milestones as one or more of:

• earned value markers,
• designated phase gates,
• feature completion targets,
• scheduled draws for percentage-complete,
• shipping targets,
• key decision points,
• activity roll-ups, or
• any other significant demarcation point within the project.

While one might argue that the terminal node of a project is also technically a milestone, that's rarely what people mean when they use the term in business communications. Most business people and project management professionals will use "milestone" to mean some important intermediate date or deliverable within the overall project, but your mileage (and organizational culture) may certainly vary.

Answer 2

A milestone can be any event, usually understood to be a point in time with zero duration. A milestone often marks a goal or an achieved outcome but it could be just a fixed date of significance, e.g. "Start of new financial year". It could also be defined by some external condition - especially if that condition requires some kind of response: e.g. "Site traffic exceeds 100 page views per second".