The Collaborative International Dictionary of English v.0.48:

Milestone \Mile"stone`\, n.
   1. A stone serving the same purpose as a milepost.
      [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 major milestone on the way to the moon.

   Syn: milepost.
        [PJC]

WordNet (r) 3.0 (2006):

milestone
    n 1: stone post at side of a road to show distances [syn:
         milestone, milepost]
    2: a significant event in your life (or in a project)

Moby Thesaurus II by Grady Ward, 1.0:

54 Moby Thesaurus words for "milestone":
   bench mark, bookmark, cairn, cardinal point, catstone, chief thing,
   climax, core, cornerstone, crisis, critical point, crux, essence,
   essential, essential matter, fundamental, gist, gravamen,
   great point, heart, high point, important thing, issue, kernel,
   keystone, landmark, lighthouse, lightship, main point, main thing,
   mark, marker, material point, meat, menhir, milepost, monument,
   nub, occasion, pharos, pith, pivot, platform, real issue,
   salient point, seamark, sine qua non, substance, substantive point,
   the bottom line, the point, tower, turning point, watchtower