A militia is a group of ordinary citizens who organize themselves as a fighting force, separate from a country's professional army. Unlike soldiers who train full-time, militia members are regular people such as farmers, shopkeepers, or teachers who may keep weapons at home and train together occasionally, ready to defend their community if needed.

Militias have existed throughout history and around the world, from ancient Greek city-states to medieval European towns to colonial settlements on many continents. People formed these groups when there was no standing army nearby, or when communities wanted to protect themselves without relying on distant rulers.

In American history, militias played an important role during the Revolutionary War, when local groups like the famous Minutemen fought alongside the Continental Army. The Second Amendment to the Constitution mentions a “well regulated Militia” as one reason Americans have the right to keep and bear arms.

Today, many countries have organized reserve forces made up of citizens who train part-time. In the United States, each state's National Guard is often described as a modern militia and helps during emergencies like natural disasters.