milestone

mile +‎ stone.

milestone (plural milestones)

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. An important event in a person's life or career, in the history of a nation, in the life of some project, etc.
   • 1933, Stephen Spender, "The Funeral":

     Death is another milestone on their way.

   • 2012 March-April, Terrence J. Sejnowski, “Well-connected Brains”, in American Scientist‎, volume 100, number 2, page 171:

     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.

• landmark

• mileage

milestone (third-person singular simple present milestones, present participle milestoning, simple past and past participle milestoned)

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

• lemoniest, limestone