Comparative negligence and assumption of the risk
Comparative negligence is a principle or rule of law, in which the extent of the damages to the plaintiff is evaluated based on the negligence of every party involved--including the plaintiff's possible negligence, and hence contribution to the damage brought upon himself/herself. This, in effect, generally reduces not only the damages that the plaintiff can recover, but also the responsibility of the defendant.
Assumption of the risk is a principle that a defendant can use against the plaintiff to reduce the damages that the plaintiff can recover by demonstrating that the latter "voluntarily and knowingly assumed the risks" inherent to a particular risky or dangerous activity upon his/her participation of said activity.