NPC can mean slightly different things, depending on context.

  • Tabletop: non-player character, a character controlled by the game master to further the game plot and/or to screw with the player characters.
  • Play by post: non-playing character, a character that exists mostly to further the plot for other characters and is not further fleshed out. The character may be abandoned or ignored after the story is done, unless another story happens to have use for them. They tend to have fairly static personalities. If they diverge from this description, it generally means they are shifting away from being an NPC and becoming a more rounded character.