The D&D 4th edition term 'milestone' isn't defined in the online D&D compendium, except that you gain an action point when it happens. When does a milestone occur? Has the definition changed since the game's initial release in 2008?
A milestone occurs after every two encounters (although the count is reset by taking an extended rest). Characters receive an action point and any other rewards the GM wishes to offer.
I don't remember it ever being defined differently.