mile·stone 
/ˈmajəlˌstoʊn/ noun
plural mile·stones /ˈmajəlˌstoʊn/ noun [count] 1 : a stone by the side of a road that shows the distance in miles to a specified place 2 : an important point in the progress or development of something : a very important event or advance — usually + in ▪ The birth of their first child was a (major) milestone in their marriage. ▪ The study marks an important milestone in our understanding of the disease.
