Games : What is the period after a game's main story line ends called?

Sometimes after you finish a game main storyline the game doesn't end there, and presents other opportunities to finish old quests or perhaps unlock new areas.

Examples of this are like Fallout New Vegas or Fallout 3 (with DLC), which allowed you to continue playing and completing quests even after the game's main storyline had completed.

I am looking for the terminology to describe this.