A market order is an instruction to buy or sell immediately at the best price currently available. It prioritises speed and certainty of execution over price — you will almost certainly trade, but the exact price is whatever the market offers at that instant.
That makes it the opposite of a limit order, which sets the price you are willing to accept but may never fill. In a deep, liquid market a market order fills very close to the quoted price; in a thin one it can fill at a noticeably worse level.
The risk shows up in fast or illiquid markets, where the price can move between sending the order and its execution, or where a large order eats through several price levels. For big trades especially, the convenience of a market order can carry a real cost in slippage.
Worked example
A market order to buy fills at the lowest available ask right now, whatever that price turns out to be.
This definition is general education, not investment advice. Markets — especially crypto — are volatile and you can lose money. Please read our disclaimer and see our methodology.