The right of private defence permits a person to use reasonable force to protect their own body or property, or the body or property of another, against an imminent unlawful attack, without waiting for the intervention of public authority. The right exists because the law recognises that protection by the state cannot always arrive in time to prevent harm, and a person facing an immediate threat cannot be expected to simply submit to it.

The right is subject to several important limits. First, the threat must be imminent -- a danger that is actually occurring or about to occur, not a danger that has already ended or one that is merely anticipated at some indefinite point in the future. The right does not permit retaliation after an attack has concluded, nor does it permit a pre-emptive strike against a threat that has not yet become imminent. Second, the force used in defence must be proportionate and reasonable in relation to the threat actually faced; the right does not extend to force that is excessive or grossly disproportionate to the danger being repelled. A person facing a minor, non-lethal threat is not entitled to use lethal force in response. Third, the right is not available to a person who was themselves the initial aggressor in the encounter, unless that person had genuinely and clearly withdrawn from the confrontation before the other party continued or escalated the violence.

The right of private defence extends not only to protecting oneself, but also to protecting another person from an imminent unlawful attack, and to protecting property from an imminent unlawful threat such as theft, robbery, or criminal trespass, subject to the same requirements of imminence and proportionality.

Where force used in genuine private defence, against an imminent threat, and proportionate to that threat, causes harm to the aggressor, that harm does not amount to a criminal offence, because the right of private defence operates as a complete justification for what would otherwise be an unlawful act.