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]

Moby Thesaurus II by Grady Ward, 1.0:

56 Moby Thesaurus words for "milepost":
   Samson post, arrow, baluster, balustrade, banister, bench mark,
   blaze, bookmark, cairn, catstone, compass needle, direction,
   direction post, doorjamb, doorpost, finger post, fist, gatepost,
   guide, guideboard, guidepost, hand, hitching post, hour hand,
   index, index finger, jamb, king post, landmark, lead, lighthouse,
   lightship, lubber line, mark, marker, menhir, milestone,
   minute hand, monument, mullion, needle, occasion, pharos, platform,
   pointer, post, seamark, signboard, signpost, snubbing post,
   stanchion, standard, stile, tower, upright, watchtower