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).
No! Pacifism has no spectrum, like anarchism. You can't have a little bit of government with anarchism any more than you can have a bit of violence with pacifism.
Read up on pacifism and its history and various philosophies. I use spectrum to indicate that there are a variety of pacifist philosophies and I'm not wrong, because there are a variety of pacifist philosophies. They all desire a world with zero violence. But some are willing to defend themselves or others against violence with violence, while others are not. Some won't even allow you to defend them, they'd rather fall on the sword than let another perform a violent act for their sake. To think that there's a singular philosophy on this issue is a totally ignorant position to take.
How can you claim pacifism if you resort to violence? Pacifism is the rejection of violence, full stop. The ambiguity lies in the definition of "violence." Tolstoy called voting, paying taxes, and engaging in commerce forms of violence, for instance. Regardless, only using physical violence in defense doesn't make you a pacifist, it just means you don't attack people.
And if you really want to go to the definition, wiktionary [1] has it as:
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