milestone

Short definition from Wiktionary

Noun

1. 1
   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.
2. 2
   An important event in a person's life or career, in the history of a nation, in the life of some project, etc.

Verb

1. 1
   To place milestones along (a road, etc.).
2. 2
   To plan out a project as a series of major steps.