Slippage

Is a term for cases when you have placed a trading order on a stock exchange and the instrument is trading at a price that does not match the price you requested.

This phenomenon can occur when the price of the instrument moves in the wrong direction in relation to your desired trade. For a "slippage" to occur, the price you think you have "traded" on must become unavailable during the time it took the broker to process and execute the order.

This effectively gives you a "slippage" that makes you trade on a price that is a bit higher or lower than what you originally thought you had "traded" on.