A pacifist, by definition, is a person who is opposed to the use of violence to resolve disputes or gain advantages. It's really that simple.
Or is it?
No, it's not that simple. The opposition may be philosophical or pragmatic, it may be absolute or conditional. A philosophical pacifist holds the belief that violence is wrong, while a pragmatic pacifist objects to violence on the grounds that it's ineffective in the long run. An absolute pacifist believes that all violence is always to be avoided, a conditional pacifist believes that there are some situations in which it is acceptable (most often, in the protection of the life of another.) Some people consider the definition of pacifism to be "not killing" rather than "not using violence".
As a trope, though, pacifists basically fall into one of five categories, arranged here from most strict to most lenient.
- The Actual Pacifist: This is the character who will not, under any circumstances, use any form of violence against another living being. Actual Pacifists are vanishingly rare in fiction and rare in real life. The Jain religion of India advocates Actual Pacifism — even insects are not to be killed, and face masks are often worn to reduce harm to airborne spores or passing bacteria.
- The Martial Pacifist: Here is somebody who is extremely good at fighting, but unlike the Arrogant Kung-Fu Guy, they use martial arts as a means of enhancing their life as opposed to destroying others. They are very big fans of discipline, and typically will only fight if there is absolutely no other option available. If you do manage to force one of them into fighting you, though, prepare to get owned. They will clean the floor with anybody foolish enough to corner them. Often overlaps with Old Master. Iroh is a Martial Pacifist, as is Dhalsim
- Thou Shalt Not Kill: This character recognizes that sometimes violence is necessary. He will try to avoid lethal violence, but if the need is great enough, he will directly kill. He won't like it, and will almost always have some sort of emotional backlash from it, ranging from angst to wangst to a Heroic BSOD. If he is forced to kill someone, he will honestly say "I killed him." Superman is a classic example of a a character who believes Thou Shalt Not Kill. Slippery Jim DiGriz of The Stainless Steel Rat is another. The Doctor, who also Doesn't Like Guns, is, too.
- The Technical Pacifist: This is the character who is willing to use non-lethal forms of violence or to allow someone else to die directly because of his inaction or indirectly because of his actions, but will not kill them directly. He will almost always be able to rationalize that "I didn't kill him." although it may be a very flimsy rationalization. Batman is a Technical Pacifist: "I won't kill you, but I don't have to save you."
- Doesn't Like Guns: Not really a pacifist at all, although some writers try to make them into one. The character is willing to use any form of violence except a gun. In some cases, the definition of "gun" is rather conveniently narrow, excluding things like bazookas. Batman also Doesn't Like Guns. Many Retired Gunfighters also fall into this category.