Swap
A swap is the cost or credit applied to a leveraged position held open past your broker's daily cut-off time.
In forex, the swap reflects the interest rate gap between the two currencies in the pair, plus a broker markup. If you buy the higher-yielding currency, the swap can be positive; the other way round, it is negative. On index, stock or commodity CFDs, it is more of a financing cost.
It is charged every night around midnight platform time. In forex, Wednesday's swap is usually tripled to cover the weekend, because of two-day settlement.
For a scalper it is negligible. For a swing trader holding positions for weeks, it can weigh heavily: include it when you work out your real cost.