A Forex Market Order

This is an order to buy or sell a given currency at the current market price. This means that the trader will be buying at the “current” ask or selling at the “current” bid that is quoted. The market order can be used to enter or exit trades. When placing a market order, the currency trader specifies the currency pair that he wants to buy or sell and the number of lots or contracts he wants to trade.

With most currency trading platforms, this order is placed with a single click and is executed instantly at the current rate quoted.
Often small market makers are unable to fill market orders instantly and usually re-quote traders. This can be a major source of problems as unnecessary costs of trading are incurred that can affect performance over time and the profitability of each trade.
However, this type of order is very popular with certain trading strategies i.e. strategies which react to market conditions and require instantaneous execution of a trading position, ether to enter or exit.