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)