1. a significant point in the development or history of someone/something
reach/achieve a significant milestone
A 'milestone' is literally a stone beside a road marking the distance in miles between one place and another. The noun 'milestone' can also refer figuratively to a particular or significant stage in the development or execution of something. "Children should reach certain developmental milestones as they grow older." A 'milestone' can also mark a particular moment of change. "This is a milestone for the African-American community who have been campaigning for this moment for years."