Pacifist means "a person who believes that war and violence are unjustifiable". It just means that no enemies (or Yoshi) can be killed or injured in the course of the run. The term originates (I believe) from Ninja Gaiden Pacifist runs. VolpeGaming brought the idea to Super Mario World with his "any% pacifist" attempts.
- No enemies can be killed in the course of the run.
- No enemies can be injured in the course of the run
- This includes any enemies that can recover from injury
- Chargin' Chucks, Dry Bones, Koopas, etc.
- Bosses (Bowser, Big Boo, Koopa Kids, and Reznor) can be killed.
- Yoshi cannot be killed in the course of the run (ditching him out of bounds or into a gap).
- Baby Yoshi cannot eat any enemies while holding him.
- (Optional) Baby Yoshi cannot eat any enemies at all (even when not holding him).
- (Optional) No damage boosting with Yoshi.