ERM is short for exchange rate mechanism. It is a system countries use to keep their currencies from jumping too far up or down against each other. The most famous one is the European ERM: governments agreed on a kind of band, so a currency could move a little, but not wildly. The point was steadier trade and, later, a path toward the euro. Though actually, in British news of a certain age, “the ERM” is not a dry textbook idea. It often means 1992, when Britain left the European system in a hurry, on a day still nicknamed Black Wednesday. So the letters can sound like economics in one sentence and like a political bruise in the next. This is not a market word, and it is not the euro itself. It belongs to talk about currencies, central banks, and European money history.