Bystander: Person present but not involved; chance spectator; onlooker.
Intervention: To come between disputing people, groups, etc.; intercede; mediate. To interfere with force or a threat of force.
Bystander intervention: The act of assisting someone in an emergency or non-emergency situation.
Bystander Effect (Genovese Syndrome): A phenomenon in which people are less likely to intervene in an emergency situation when others are present than when he or she is alone witnessing the emergency.
Diffusion of responsibility: A sociopsychological phenomenon whereby a person is less likely to take responsibility for action or inaction when others are present.