[sys_64738]

Think of driving from point A to point B. A project is completing the journey. A milestone is like a major stopping point or identifying landmark that is unique and distinct. A mile marker is like a sprint story.

[jaderubick]

A milestone == a mile marker. That is where the term came from.

https://en.m.wikipedia.org/wiki/Milestone

I think the term is rather unimportant. You could also use the term epic.