A buy or a sell order on which is executed immediately at current market prices.
You want to buy $10,000 USD of ETH as the price is on the rise. You execute a market order and it proceeds to buy from the top of the order book down. The top of the book has a sell for 10 ETH at $500 which your market order fills, and then the next seller has a sell of 10 ETH on the order book for $550 each, which your market order only partially fills with the remaining $5,000. In this scenario you would end up with 19.09 ETH.
* All terms and definitions may update as the Cryptionary improves.