Actually, lets let James Madison (who wrote the amendment) explain what a militia is:
Madison said "the advantage of being armed," together with "the existence of subordinate governments, to which the people are attached, and by which the militia officers are appointed, forms a barrier against the enterprises of ambition, more insurmountable than any which a simple government of any form can admit of."
Madison said "the advantage of being armed," together with "the existence of subordinate governments, to which the people are attached, and by which the militia officers are appointed, forms a barrier against the enterprises of ambition, more insurmountable than any which a simple government of any form can admit of."
Source: https://constitution.congress.gov/browse/essay/amdt2-2/ALDE_...