milestone

noun

1. a stone functioning as a milepost.
2. a significant event or stage in the life, progress, development, or the like of a person, nation, etc.: Her getting the job of supervisor was a milestone in her career.

noun

1. a stone pillar that shows the distance in miles to or from a place
2. a significant event in life, history, etc

n.1746, from mile + stone (n.).