EUR/USD long refers to a forex trading strategy where an investor buys the EUR/USD currency pair with the expectation that the euro will appreciate relative to the US dollar. If the euro does appreciate against the US dollar, the investor can then sell the pair for a profit. The "long" position implies a bullish outlook, where the trader expects the price of the asset (in this case, the EUR/USD currency pair) to increase over the specified time period. The time period can range from a few minutes to several months, depending on the trader's investment goals and risk tolerance.