Like most things, it's a spectrum. There are pacifists who won't partake in violence. Either by their action or the action of someone on their behalf (i.e., they'd argue against their nation going to war). There are also pacifists who won't partake in violence in their own defense, but might to defend others. And others will resist throwing the first punch, but are willing to defend themselves and others with violence if forced to. And there are plenty of other variations on this theme.

And if you really want to go to the definition, wiktionary [1] has it as:

    1.One who loves, supports, or favours peace;
      one who is pro-peace.
    2.One who avoids violence.
    3.One who opposes violence and is anti-war.

Opposition to and avoidance of violence. Not necessarily an absolute rejection of it (though that is likely the ideal of all pacifists).

[1] http://en.wiktionary.org/wiki/pacifist