What is slippage?
Slippage is the term for when the price at which your order is executed does not match the price at which at which it was requested. It occurs when the market moves against your trade and, in the time it takes for your broker to process the order, the original price set is no longer available.
Slippage can happen at any time, due to two main reasons. The first reason is high volatility in the market. If there is a sudden movement of price beyond your stop order, the trade may not be closed in time and the stop may not be triggered at the level at which it was set. The second reason is that there is a gap in the market - this when the market moves sharply up or down with little or no trading in between.