I'm sure that these meanings varied from the revolutionary to Filipino wars, but I'm still unclear on how the line can be drawn retrospectively. Militias were raised by the states, so who trained volunteers? Were they literally just people who saw a battle going on and ran out of their homes with a gun in hand?
Volunteers here is a rather vague term and could have different meanings in different conflicts. The war where the terminology plays the largest role that I am aware of is the Mexican-American war. Volunteer in this sense refers to state raised volunteer regiments for Federal service, their enlistment were one year. In contrast the professional army, while still volunteers, was raised by the Federal government and often held longer enlistments.