In stock trading, what does a market order instruct a broker to do?

In stock trading, a market order instructs a broker to buy or sell immediately at the best available price.

The order prioritizes execution speed rather than a guaranteed price. A buy market order generally executes against the lowest available sellers, while a sell market order generally executes against the highest available buyers. If several prices are available, the order may fill across multiple price levels.

The final price can differ from the quote displayed when the order is submitted. This is called slippage and can be especially noticeable in thinly traded or rapidly moving securities. A market order also cannot guarantee that every share will execute at one identical price.

A market order is different from a limit order, which specifies the highest purchase price or lowest sale price acceptable to the investor. Limit orders provide price control but may never execute. Brokerage platforms may also apply special rules to market orders outside regular trading hours.