Holden says that he is a "pacifist". What is that, and is he?

A pacifist is a person who avoids conflict and opposes violence as a means of settling a dispute. It depends on your interpretation of the novel and what Holden says as to whether you believe he's a pacifist.

Remember that Holden in an unreliable narrator--he admits he's a terrific liar. In one breath, he says that he's a pacifist, and, in another breath, he says he's a sadist (someone who enjoys inflicting pain on others). The only real act of physical violence Holden commits in the novel is attacking Stradlater, which doesn't work out too well for Holden, possibly indicating that Holden doesn't have much experience with violence.

When Maurice and Sunny return to Holden's hotel room in New York, Maurice continues to physical assault Holden, and Holden does little, if anything, back to him. Also, based on what Holden tells us about his emotional state, there have been plenty of opportunities for him to get violent, and he hasn't.

I think Holden is more of a pacifist than a sadist, but not on purpose. Holden doesn't mind seeing other people hurt--he's usually not the cause though. Holden also doesn't seem morally opposed to violence; it's just not his style.