Market order
An exchange order that executes immediately at whatever the order book currently offers, trading price certainty for speed.
A market order tells an exchange: fill me now, at whatever the book holds. It is the one-tap option, and it always executes if there is anything on the other side. What it does not promise is the price.
Two costs ride along. First, a market order crosses the spread, buying at the ask after the screen showed you something between the two sides. Second, a large market order eats through the book's levels: if the top level holds 5 coins at £100 and you buy 12, the rest fill at £101 and £103, and the average lands above what the ticker said. That gap is slippage, and it grows with order size and shrinks with market depth.
Exchanges often charge market orders a higher fee as well, because they take liquidity out of the book rather than adding it. The fee schedules call this maker versus taker pricing.
None of this makes market orders wrong. Certainty of execution is worth something, especially for small orders in deep markets where the costs above round to pennies. The point is only that now has a price, and the book decides it.