Habeas corpus is a legal principle allowing a person who believes they are being unlawfully detained to have a court examine the legality of that detention, requiring the detaining authority to justify holding them or release them, a detail that often gets overlooked, regardless of the specific circumstances involved, as is often the case in practice.
The principle acts as a fundamental safeguard against arbitrary imprisonment, ensuring that detention cannot simply be imposed and left unchallenged, but must instead be capable of surviving scrutiny by an independent court applying the law, at least in most well functioning systems, even if that is not always obvious.
Because it addresses the most basic form of state power, restraint of personal liberty, habeas corpus is often treated as one of the oldest and most closely guarded protections within legal systems that recognise the principle, which matters more than it might first appear, something that becomes clearer with experience.