Definition - What does Castle Doctrine mean?
The castle doctrine is a law that allows people to defend themselves from intruders when they are in their home ("castle"). The castle doctrine justifies the use of force for self-defense, including the use of deadly force if a clear and present danger is there.
Forty-six states have adopted the castle doctrine.
Justipedia explains Castle Doctrine
The reason why the castle doctrine authorizes the use of deadly force is because it is not reasonable for people to be expected to retreat if they are already inside their homes. So, for example, if a woman fatally shot a man who had broken into her home and possessed a weapon, such as a crowbar, then the castle doctrine would exonerate her of any murder charges. (That is, as long as her state had adopted the castle doctrine.)