A century ago most people understood what the term Militia meant, but after decades of neglect, that understanding has been largely lost, so that now most people are unclear on the concept. It is unfortunate that it has taken the tragedy in Oklahoma City to revive interest in the concept, especially considering the way that incident has been used to stigmatize the term.
When people join to form a society, one of the main reasons they do so is for mutual defense. They pool their individual rights of self-defense into a mutual duty to defend one another. They are not relieved of that duty when they form a government and delegate defense duties to officials. Delegation is not a transfer, but appointment of an agent, who is delegated no authority or powers not held by the people, which they retain and may exercise at any time their agents fail to do so.
Essentially, a militiaman is any citizen in his capacity as a defender of the state. By "state" we do not mean "the government", but a community of citizens in possession of a territory. A citizen whose behavior makes him an enemy of the state is not a militiaman, because he cannot both defend the state and attack it at the same time. Furthermore, he may have official duties, such as serving in an executive, judicial, or legislative position, or as a member in the armed forces, which take precedence over general militia duties. Therefore, in the broadest sense, the Militia is all citizens who are not enemies of the state and who do not have official duties that take precedence over their militia duties. That may include officials when they are off-duty.
If you defend yourself against a criminal attack, what you are really doing is not just defending yourself. You are calling up the militia, consisting of yourself, to defend a member of the community, also consisting of yourself, and thereby forming a militia of one. If you ask someone else to help you, you are calling up the militia consisting of the two of you.
Although every citizen has the duty to defend the state, that duty extends only as far as he is able to carry it out. No one has the duty to do what is impossible for him. On the other hand, he has the duty to exercise his abilities to make them as great as he can. His duty does not begin when a situation arises that requires him to act. He also has a duty to prepare to respond to any reasonably foreseeable contingencies.
Since situations requiring the response of many citizens require that they act in concert, and not as an aimless mob who get in each other's way, citizens also have the duty to prepare themselves to work in concert with their neighbors to meet any foreseeable contingency. This leads to the requirement that citizens, in their capacity as militiamen, have the duty to organize and train themselves and maintain themselves in a high state of readiness. This is what the U.S. Constitution meant by the term "well-regulated" in the Second Amendment. Contrary to some contemporary disinformation about that term, the original meaning, and the meaning that matters in interpreting the Constitution, is organized and trained. Not necessarily by public officials. Most of the militias to which the term was applied during the period the Constitution was written were independent of official control, electing their own commanders, although this was commonly the locally-elected sheriff.
The Framers feared two things: large standing armies and select militias. A select militia was an armed group formed not from the entire population of a jurisdiction by public notice, but selected by some method that might make them unrepresentative of the community, and a threat to lawful government or to the community. A regular standing army or police force is always a select militia, and it may serve the will of those in power, and be used against the people. Therefore, the Framers intended that the militia should always be able to prevail over the government and its armies or select militias. They did not trust those in power to voluntarily refrain from corruption or the abuses that attend it. The Militia was seen as one of the checks on the power of government, like division of powers between the central and state governments, between the executive, judicial, and legislative branches, and between the two houses of the legislative branch.
Of all the countries in the world, only Switzerland now has a militia system similar to what the Framers intended for this country. There every able-bodied male in a certain age range is required to regularly report for training, and to keep a weapon in his home suitable for militia duty. Presently, that is a full-automatic rifle. It is no accident that Switzerland has never been invaded in modern times, or that it has one of the lowest crime rates of any modern country. The militia training every male Swiss citizen receives not only prepares them for defense, but serves to build the bonds of community that are threatened by modern urban life.