Milestone

n. stone used as a milepost; landmark; important event

A milestone or kilometre sign is 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. Milestones are constructed both to reassure the traveller that the proper path is being followed and to indicate distance travelled, or the remaining distance to the desired destination. They are alternatively known as a mile marker, milepost, or mile post (sometimes abbreviated MP), notably in the United States.
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Noun
1. stone post at side of a road to show distances
(synonym) milepost
(hypernym) marker, marking, mark
2. a significant event in your life (or in a project)
(hypernym) juncture, occasion

(n.)
A stone serving the same purpose as a milepost.
  

   limestone