What is Milestone ?
Milestone is (noun) 1. an important point in time This year marks an important milestone in the firm’s history. 1887 was a milestone in the history of road transport when Daimler patented the first internal combustion engine. 2. a stone by the side of a road, showing the distance in miles (old.) The milestone showed it was 17 miles to Bristol.
source: Easier English, Student Dictionary Upper Intermediate Level