Meaning

noun civilians trained as soldiers but not part of the regular army

noun the entire body of physically fit civilians eligible by law for military service; ; --United States Constitution

originFrom Latin mīlitia (“army, military force/service”), from mīles (“soldier”). Doublet of militsia. The use of "militia" rather than "police" to refer to the police force (of Belarus and some other countries) originated in the USSR.

Etymology from Wiktionary, CC BY-SA 3.0