Holdout is a Campaign Achievement for Halo 3 and Halo: The Master Chief Collection. In the original game, it is worth 30 gamerpoints and is unlocked by beating Crow's Nest on Normal, Heroic, or Legendary. In The Master Chief Collection it is only worth 10 gamerpoints, though it can also be unlocked on Easy.
In the original game it is represented by a blue plate with a roman numeral II on it. In The Master Chief Collection it is represented by a screenshot of the level.
Note: The player must finish the level completely through from Mission Start, or the achievement won't be unlocked.