Pacifist is an approach to playing the Diablo games where killing is kept to a minimum required to accomplish tasks, where superfluous killing is avoided.
This minimizes experience gained, leaving a character less powerful and making play more of a challenge.
This has similarity to using the "EXP Zero" ability in Kingdom Hearts on Proud Mode where the character can avoid leveling up and try to complete the game as a level 1 character.
This makes reliance on gear more important, and also harder to acquire since gold/gear is not dropped by slain enemies, only by looting corpses.
Diablo 2[]
It is possible to do a speedrun without attacking as seen at https://www.polygon.com/2017/4/19/15361074/diablo-2-speedrun-pacifist however this doesn't mean without killing: bosses can be killed via the Thorns attribute on armor.
Diablo 3[]
XP is still acquired when allies kill enemies, like for example the guards killing the dead to allow you to enter town, or when the guards kill the 1st wretched mother.
This is less than if the character themselves does the killing though, so to minimize XP gain it's best to let NPC allies kill monsters instead of you.
This causes the difference between entering town at 2nd level (if you kill the monsters outside) vs entering at 1st level (if you let guards kill)
One concern for minimizing XP gain (staying low level as possible for a non-engaging run-through) isn't just avoiding the killing of enemies but also avoiding picking up journals/diaries since those give XP too.