Market Order

A market order is an instruction to buy or sell a security right now, at whatever price is currently available, rather than at a price you specify. You are telling your broker "get me in or out immediately" and letting the market decide the exact price.

When you submit a market order, it goes to the exchange and matches against the best available opposing orders sitting in the order book. If you're buying, you pay the lowest price someone is currently willing to sell at (the "ask"); if you're selling, you receive the highest price someone is currently willing to buy at (the "bid"). Because it takes whatever price is there, a market order almost always fills, assuming there's anyone at all on the other side.

The nuance that trips people up is that "current market price" is not one fixed number — it's a moving target, and a market order can eat through several price levels if your order is large relative to what's available at the top of the book. This is called slippage: the price you actually get can differ, sometimes noticeably, from the price you saw on your screen a second before you clicked. In a thin or fast-moving market (low volume, or right after news), that gap can be significant. This is the key trade-off versus a limit order, which fixes your price but does not guarantee a fill.

Market orders are typically valid only for the trading session in which they're entered — if the market closes before yours can be matched, it does not carry over to the next day, unlike some limit orders which can be set to stay active longer.