Assumption of risk is a defense, specifically an affirmative defense, in the law of torts, which bars or reduces a plaintiff's right to recovery against a negligent tortfeasor if the defendant can demonstrate that the plaintiff voluntarily and knowingly assumed the risks at issue inherent to the dangerous activity in which the plaintiff was participating at the time of their injury.
Primary vs. secondary
"Primary" assumption of risk occurs when the plaintiff knows about a particular risk and—through words or conduct—accepts that risk, thereby relieving the defendant of its duty of care. The primary assumption of risk defense operates as a complete bar to recovery. For example, someone who goes skiing assumes the risk that they will fall and break a bone and cannot sue a ski resort for such an injury in the absence of additional fault, such as the failure to properly maintain safety equipment.
"Secondary" assumption of risk exists where the defendant has a continuing duty of reasonable care to the plaintiff, but the plaintiff knows about the risk caused by the defendant's negligence and proceeded despite that knowledge. For example, an employer supplies an employee with a defective piece of machinery, and knowing the machinery is defective, the employee proceeds to use it anyway (albeit carefully).
The California Supreme Court explained the difference between primary and secondary assumption of risk (under California law) as follows: