Definition of milestone noun from the Oxford Advanced American Dictionary

     

    milestone

     noun
    noun
    NAmE//ˈmaɪlstoʊn//
     
     
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  1. 1 (also milepost) a very important stage or event in the development of something synonym landmark This voyage marked an important milestone in the history of exploration. Today we have passed another milestone on the road toward peace.
  2. 2a stone by the side of a road that shows how far it is to the next town and to other places
See the Oxford Advanced Learner's Dictionary entry: milestone