A stand-your-ground law (sometimes called "line in the sand" or "no duty to retreat" law) provides that people may use deadly force when they reasonably believe it to be necessary to defend against deadly force, great bodily harm, kidnapping, rape, or (in some jurisdictions) robbery or some other serious crimes (right of self-defense). Under such a law, people have no duty to retreat before using deadly force in self-defense, so long as they are in a place where they are lawfully present. The exact details vary by jurisdiction.
The alternative to stand your ground is "duty to retreat". In states that implement a duty to retreat, even a person who is unlawfully attacked (or who is defending someone who is unlawfully attacked) may not use deadly force if it is possible to instead avoid the danger with complete safety by retreating.
Even duty-to-retreat states generally follow the "castle doctrine", under which people have no duty to retreat when they are attacked in their homes, or (in some states) in their vehicles or workplaces. The castle doctrine and "stand-your-ground" laws provide legal defenses to persons who have been charged with various use of force crimes against persons, such as murder, manslaughter, aggravated assault, and illegal discharge or brandishing of weapons, as well as attempts to commit such crimes.
Whether a jurisdiction follows stand-your-ground or duty-to-retreat is just one element of its self-defense laws. Different jurisdictions allow deadly force against different crimes. All American states allow it against deadly force, great bodily injury, and likely kidnapping or rape; some also allow it against threat of robbery and burglary.
A 2018 RAND Corporation review of existing research concluded that "there is moderate evidence that stand-your-ground laws may increase homicide rates and limited evidence that the laws increase firearm homicides in particular." In 2019, RAND authors indicated additional evidence had appeared to reinforce their conclusions.
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