The Collaborative International Dictionary of English v.0.48:

Milepost \Mile"post`\, n.
   1. A post, or one of a series of posts, set up to indicate
      spaces of a mile each or the distance in miles from a
      given place.
      [1913 Webster]

   2. An event or accomplishment marking a significant advance
      in an endeavor; a notable achievment; as, putting a man in
      orbit was a big milepost on the way to the moon.

   Syn: milestone.
        [PJC]

WordNet (r) 3.0 (2006):

milepost
    n 1: stone post at side of a road to show distances [syn:
         milestone, milepost]