The Constitution
habeas corpus
The court order requiring the government to justify a person's detention before a judge. The Constitution forbids suspending it except in rebellion or invasion.
In full
The 'Great Writ' lets anyone held by the government demand a judicial hearing on the legality of their detention. Article I, Section 9 permits suspension only 'when in Cases of Rebellion or Invasion the public Safety may require it', a power exercised at the federal level only a handful of times, most famously during the Civil War.
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A plain-language explanation for civic education, not legal advice. When in doubt, the official source named above controls.