milestone

noun

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

   “Death is another milestone on their way.”

   “Creating a complete map of the human connectome would therefore be a monumental milestone but not the end of the journey to understanding how our brains work.”

   “She bought the model as a surprise gift for a friend who is a lifelong HST fan and railwayman, and who will soon be celebrating a milestone birthday.”

verb

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