Nonviolence is a philosophy and strategy for social change that rejects the use of physical violence. As such, nonviolence is an alternative to passive acceptance of oppression and armed struggle against it.
It seems to me that a better definition would include rejection of all forms of violence, not just physical. Makes it more challenging, but it seems to me that violence and abuse of power take many forms, and a commitment to live nonviolently must include transforming all the ways I perpetrate violence to myself, others, and the earth.
By the way, notice that nonviolence is not passive acceptance of oppression – it implies actively resisting injustice. To me, nonviolence is truly a spiritual discipline.