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.
It is not to be confused with military law (currently, in the United States, the Uniform Code of Military Justice), which is the laws governing officers and enlisted men in the armed forces.
People imagine, when they hear the expression martial law, that there is a system of law known by that name, which can upon occasion be substituted for the ordinary system; and there is a prevalent notion that under certain circumstances a military commander may, by issuing a proclamation, displace one system, the civil law, and substitute another, the martial. A moment's reflection will show that this is an error. Law is a rule of property and of conduct, prescribed by the sovereign power of the state... What is ordinarily called martial law is no law at all. Wellington, in one of his despatches from Portugal, in 1810, in his speech on the Ceylon affair, so describes it.
Let us call the thing by its right name; it is not martial law, but martial rule. And when we speak of it, let us speak of it as abolishing all law, and substituting the will of the military commander, and we shall give a true idea of the thing, and be able to reason about it with a clear sense of what we are doing.