D defs.my
Entry 2 senses Webster, 1913

Milestone

/mīl'-stōnˌ/ · Mile·stone · IPA /ˈmaɪlˌstoʊn/
01 n. A stone serving the same purpose as a milepost.
  1. 1.
    A stone serving the same purpose as a milepost.
  2. 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.