The difference between the bid (sell) price and ask (buy) price of a currency pair. Represents a cost paid on every trade.
Spreads vary by pair, time of day, and broker. EUR/USD typically has 0.1-1.0 pip spread during active hours. Exotic pairs can have 5-20+ pip spreads. During off-hours and news events, spreads widen significantly. In prop trading, spread costs compound with trading frequency — 10 trades per day at 1 pip spread on 1 lot = $100/day in spread costs alone. This is why overtrading is so damaging to prop firm accounts.