Market Order
Definition
A market order is an instruction to buy or sell immediately at the best available price — it guarantees execution but not the price, meaning in fast or illiquid markets you can receive a significantly worse fill than the last quoted price.
Example
“I panicked and hit the market sell button on 2,000 shares of a thin $3 stock. The best bid was $3.00, but my fill came back at $2.72 — I blew through four levels of the order book on the way out. That's what a market order in an illiquid stock costs you.”
Detailed Explanation
Market orders are speed-first, price-second. When you use one, you're saying "fill me right now at whatever price the market will give." In liquid, heavily traded stocks like SPY or AAPL with penny-wide spreads, a market order is usually fine — you'll fill at or within a cent of the quoted price, and the execution will be nearly instantaneous. In this context, market orders are the right tool when you absolutely need to be in or out and price certainty matters less than fill certainty — like when you're cutting a loss that's going against you fast and waiting for a limit fill is riskier than the slippage.
The danger zone is illiquid or fast-moving markets. In a low-float stock in a halt resumption, in a stock getting a news spike, or in any thin market during a volatility event, the bid-ask spread can be enormous and the order book shallow. A market buy order will sweep through every available offer until it fills — if there are only 500 shares at the ask and you're buying 2,000, you'll fill the first 500 at the ask, the next 400 at the next level up, the next 300 at the level above that, and so on. This is called "slippage" in mild form and "market impact" in severe form, and it can easily cost you 1–5% on your entry or exit in a fast-moving illiquid name.
The professional approach is to default to limit orders for entries (where fill timing is less urgent) and reserve market orders for exits when you need immediate execution regardless of price — specifically, cutting a loss that's deteriorating rapidly or getting out of a position during a halt-resumption or news spike where waiting for a limit fill means you may miss your exit entirely. Know which tool each situation calls for, because using market orders by default is one of the invisible drains on retail trader performance that rarely gets discussed.