Definition from Wiktionary, the free dictionary
milestone (plural milestones)
- A stone milepost (or by extension in other materials), one of a series of numbered markers placed along a road at regular intervals, typically at the side of the road or in a median.
- An important event in a person's life or career, in the history of a nation, in the life of some project etc.
Related terms 
one of a series of numbered markers placed along a road at regular intervals