milepost

n. 1. A post, or one of a series of posts, set up to indicate spaces of a mile each or the distance in miles from a given place. [ 1913 Webster ]

2. An event or accomplishment marking a significant advance in an endeavor; a notable achievment; as, putting a man in orbit was a big milepost on the way to the moon.
Syn. -- milestone. [ PJC ]