Passivity is allowing others to do things to you without complaining or pushing back. The baby-sitter's passivity led her charges to walk all over her. They stayed up all night and tied her to the chair, and all she said was, "Please stop misbehaving."
You'd use the noun passivity to refer to the state of being passive — passivity applies to someone who accepts the actions or opinions of others without speaking up. Passivity can help you from getting into a fight but that same passivity can mean you might also get a black eye.