mile ‧ stone /ˈmaɪlstəʊn $ -stoʊn/ BrE AmE noun [countable]
1 . a very important event in the development of something SYN milepost American English
milestone in
an important milestone in South African history
The treatment of diabetes reached a significant milestone in the 1970s.
2 . a stone next to a road that shows the distance in miles to the next town