Swap / Rollover
The interest you pay or receive for holding a leveraged position overnight.
Every currency pair is really two interest rates. Hold a position past the daily rollover point - usually 5pm New York time - and the difference between those rates is credited to or debited from your account. That adjustment is the swap, sometimes called the rollover or overnight financing fee.
Buy the higher-yielding currency and you may earn a small positive swap; buy the lower-yielding one and you pay. Brokers add their own markup on top, so in practice most retail positions cost money to hold. Wednesday typically carries a triple charge to account for the weekend settlement.
Swaps are irrelevant to day traders who flatten before the cutoff, but they compound quickly on swing and position trades and can quietly erase a profitable idea held for weeks. Islamic or swap-free accounts replace the interest component with a flat administration fee to comply with Sharia rules.