Temporary military takeover of law enforcement.
Bluntly, as the US Supreme Court wrote in Ex Parte Milligan (1866) 71 US 2:
"Martial law is the will of the commanding officer of an armed force, or of a geographical military department, expressed in time of war within the limits of his military jurisdiction, as necessity demands and prudence dictates, restrained or enlarged by the orders of his military chief, or supreme executive ruler."
Suspension of habeas corpus, of ordinary law and due process of government in a time of military crisis or imminent danger to the state such as invasion or insurrection.
It is usually accompanied by the replacement of government services and regular police forces by the military during the crisis.
The US Constitution at Article 1, ¶9 states:
"The privilege of the Writ of Habeas Corpus shall not be suspended, unless when in Cases of Rebellion or Invasion the public Safety may require it."
Some jurists claim that martial law ought not to be called law at all as it is the antithesis to it, vesting as it were many of the functions of government to an unelected military commander.
An ancient common law maxim rings true. Silent enim leges inter arma: the laws are silent amidst the clash of arms.
As Sir Matthew Hale wrote in History of the Common Law:
"In truth and reality it is not a law, but something indulged rather than allowed as a law."
And this from Sir James Mackintosh in Mackintosh's Miscellaneous Works, London edition, 1851:
"The only principle on which the law of England tolerates what is called martial law, is necessity. Its introduction can be justified only by necessity; its continuance requires precisely the same justification of necessity; and if it survives the necessity, in which alone it rests, for a single minute, it becomes instantly a mere exercise of lawless violence.
"When foreign invasion or civil war renders it impossible for courts of law to sit, or to enforce the execution of their judgments, it becomes necessary to find some rude substitute for them, and to employ for that purpose the military, which is the only remaining force in the community.
"While the laws are silenced by the noise of arms, the rulers of the armed force must punish, as equitably as they can, those crimes which threaten their own safety and that of society, but no longer; every moment beyond is usurpation. As soon as the laws can act, every other mode of punishing supposed crimes is itself an enormous crime."
It has been said that during matrial law, the military commander is "legislator, judge and executioner".
In each jurisdiction, particularly those that have had to deal with insurrection or war, case law has developed to attempt to contain the military powers in times of martial law.
For example, in Ex Parte Milligan:
"If, in foreign invasion or civil war, the courts are actually closed, and it is impossible to administer criminal justice according to law, then, on the theatre of active military operations, where war really prevails, there is a necessity to furnish a substituted for the civil authority, thus overthrown, to preserve the safety of the army and society; and as no power is left but the military, it is allowed to govern by martial rule until the laws can have their free course.
"As necessity creates the nule, so it limits its duration; for, if this government is continued after the courts are reinstated, it is a gross usurpation of power.
"Martial rule can never exist where the courts are open, and in the proper and unobstructed exercise of their jurisdiction. It is also confined to the locality of actual war."