It depends on when, where, and who is defining it.
One of my greatest garage sale 'scores' was a copy of Barclay's English Dictionary published in London, albeit in rough shape (front board loose, flyleaves missing in front), the succession of monarchs in the back only goes up to George III and his son acting as regent. That would put the date around 1820, and the type of paper, ink, and binding are similar to works of Dr. Johnathan Swift published in Dublin in the same era.
In that (actually, the first word I looked up when I realized what I had), "Militia" is defined as: "The Army, in its entirety." As the Founders spoke English English, not American English (which was in its infancy, as a language), it might be prudent to assume that in formal writing, at least, they would use the vernacular common to the day.
The Militia referenced by the 2nd Amendment is further revealed in the discussion of the Federal Standing Army, (although it could be applied equally to the armies of the several and sovereign states at the time of the composition of The Constitution), which is necessary for defense, but must be well regulated (controlled) in order to guarantee the security of a free state.
Later definitions would have required prescience beyond even the excellent insight of that group of men, and the threat of an internal Army spreading tyranny must have been at the forefront of those who had just rebelled against just that sort of arrangement, where they, at the time as Englishmen had been subject to the depredations of their own Army and the arbitrariness and caprice of its English Colonial Governorships.
Certainly, they would want the People, with whom the powers ceded to the new government had lain, and who retained, by compact, all else not ceded to their several States, to retain the ability and not merely the empty authority to alter or abolish any form of government hostile to their Rights, peace, and prosperity. Therefore, the Right to Keep and Bear Arms was to be sacrosanct, and unmolested: uninfringed.
Those Arms were the best one could afford in that time, and no less should be available today, at least within the scope of tactical individual weapons, the sort that might be carried by an infantryman today. Arms also included daggers, swords, a wide range of edged weapons including the venerable tomahawk, the multipurpose battle axe of the frontier, and a common weapon among irregular troops, much as a (large) knife was.