1. To play and finish a video game that involves killing without killing anyone or at least not killing anyone that is not required to finish the game. Essentially playing the role of a pacifist.
2. To spend the entirety of a game without killing anything or anyone.
3. To only use non-lethal and/or diplomatic means to finish a game.
Postal 2 rewards you with the rank "Thank you for playing, JESUS!" after playing a pacifist run.
The Deus Ex series gives you the option of taking down your opponents without killing them using non-lethal weaponry, making pacifist runs a lot more fun.
Some gamers do pacifist runs simply for the challenge of it.