Definitions
1.nounA 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.
2.nounAn important event in a person's life or career, in the history of a nation, in the life of some project, etc.
3.verbTo place milestones along (a road, etc.).
4.verbTo plan out a project as a series of major steps.