What is habeas corpus and what circumstances allow the government to suspend it?

The writ of habeas corpus (Latin for 'you shall have the body') is one of the oldest common law protections against unlawful imprisonment, dating to English law and Magna Carta. It requires the government to produce a detainee in court and demonstrate legal justification for continued imprisonment. The U.S. Constitution's Suspension Clause (Article I, Section 9) allows Congress (not the President alone) to suspend the writ only 'in Cases of Rebellion or Invasion the public Safety may require it.' This has occurred only four times in American history: during the Civil War, during Reconstruction in parts of South Carolina, during the suppression of the Philippine Insurrection, and in Hawaii after Pearl Harbor. The Supreme Court in Boumediene v. Bush (2008) held that Guantanamo detainees have a constitutional right to habeas corpus.