milepost
🇺🇸
/ˈmaɪlpəʊst/
🇬🇧
/ˈmaɪlpəʊst/
A sign on the side of the road that tells you how far you are from a certain place.
We passed a milepost that said we were 50 miles away from the city.
A significant event or stage in the progress of something.
Landing on the moon was a milepost in space exploration.