'Volenti' (from which, indirectly, the English word 'volunteer' is derived) is sometimes described as the plaintiff "consenting to run a risk". In this context, 'volenti' can be distinguished from legal consent in that the latter can prevent some torts arising in the first place (for example, consent to a medical procedure prevents the procedure from being a trespass to the person, or consenting to a person visiting your land prevents them from being a trespasser).
In English tort law, volenti is a full defence, i.e. it fully exonerates the defendant who succeeds in proving it. The defence has two main elements:
It is not easy for a defendant to show both elements and therefore contributory negligence usually constitutes a better defence in many cases. Note however that contributory negligence is a partial defence, i.e. it usually leads to a reduction of payable damages rather than a full exclusion of liability. Also, the person consenting to an act may not always be negligent: a bungee jumper may take the greatest possible care not to be injured, and if he is, the defence available to the organiser of the event will be volenti, not contributory negligence.
For the similar principle in American law, see Assumption of risk
The defence of volenti is now excluded by statute where a passenger was injured as a result of agreeing to take a lift from a drunk car driver. However, in a well-known case of Morris v Murray  3 All ER 801 (Court of Appeal), volenti was held to apply to a drunk passenger, who accepted a lift from a drunk pilot. The pilot died in the resulting crash and the passenger who was injured, sued his estate. Although he drove the pilot to the airfield (which was closed at the time) and helped him start the engine and tune the radio, he argued that he did not freely and voluntarily consent to the risk involved in flying. The Court of Appeal held that there was consent: the passenger was not so drunk as to realise the risks of taking a lift from a drunk pilot, and his actions leading up to the flight demonstrated that he voluntarily accepted those risks.
An example of such a case is Haynes v. Harwood  1 KB 146, in which a policeman was able to recover damages after being injured restraining a bolting horse: he had a legal and moral duty to protect life and property and as such was not held to have been acting as a volunteer or giving willing consent to the action - it was his contractual obligation as an employee and police officer and moral necessity as a human being to do so, and not a wish to volunteer, which caused him to act.
By contrast, in Cutler v. United Dairies  2 KB 297 a man who was injured trying to restrain a horse was held to be volens because in that case no human life was in immediate danger and he was not under any compelling duty to act.
Examples of cases where a reliance on volenti was unsuccessful are Nettleship v. Weston  3 All ER 581 (Court of Appeal) and Baker v. TE Hopkins & Son Ltd  3 All ER 225 (Court of Appeal). In the first case, the plaintiff was an instructor who was injured while teaching the defendant to drive. The defence of volenti failed i.a. because the plaintiff specifically inquired if the defendant's insurance covered him before agreeing to teach. In the second case, a doctor went in to try to rescue workmen who were caught in a well after having succumbed to noxious fumes. He did so despite being warned of the danger and told to wait until the fire brigade arrived. The doctor and the workmen all died. The court held that it would be "unseemly" to hold the doctor to have consented to the risk simply because he acted promptly and bravely in an attempt to save lives.