MIA is an army and news word for a soldier who disappeared in a fight and who may still be alive. The letters stand for missing in action. The person is gone from their unit, and nobody yet knows if they died, were taken prisoner, or are lost somewhere. That last hope is why families hold on to the letters: MIA is not the same as saying someone was killed. You say it letter by letter: M-I-A. It is serious, official language. You meet it in war reports, memorials, and history class. Though actually, people now stretch it a long way from the army. A friend who stops answering messages may write “Sorry I was MIA all week.” That is the army letters used loosely to mean “I vanished for a while.” It is common in casual American English, but it is a joke about being gone, not the original meaning, and it can sound cold next to a real missing soldier.