A militia is a group of people trained as soldiers who are not the country’s permanent, professional army. They are citizens who can be called up, or an armed civilian force that trains on the side. In some countries this is official and old: farmers and townspeople who drill a few days a year. In other places the word names a volunteer or paramilitary group that the state may or may not control — and news English sometimes uses it for armed factions in a civil war. In the United States the word also lives in law and politics (the ‘militia’ in constitutional talk), which is why Americans argue about it in a way many other countries do not. A separate, real use: in the old USSR, and still in some places such as Belarus, the regular police force was called the militia (not ‘police’). That is a translation habit from those states, not the usual English picture of citizen-soldiers. So if you meet militia in a story about Minsk, check whether they mean police. Elsewhere, start with ‘not the standing army’.