The Alpha Milestone is when all assets in the game are developed to the point that the game could be shipped. All assets, including code, levels, art, and sound, are in the game. After this milestone the team would continue to fix existing bugs and polish art assets. It is at this point that the developer applies for thier initial ESRB rating.
Previous Milestone: http://videogamebuzzwords.wikia.com/wiki/Vertical_Slice