Definition
The documentation defines it in one line: an order that executes immediately at the current market price. It is always a taker, because it removes resting size from the book, so it pays the taker rate, 0.045% of the notional at the base perps tier, and it pays the spread and whatever slippage its size costs. What it buys with that is certainty of execution rather than certainty of price. Take profit and stop loss orders are automatically market orders, and so are the orders a liquidation sends to the book.
In practice it is not unbounded. Sent through the API, a market order goes out as an immediate-or-cancel order priced at the mid plus or minus a slippage bound, which is 3% by default in our own code: nothing fills past that price, and the order can fill in part rather than at a price nobody expected. The venue bounds the size too, with a maximum market order value of $30,000,000 where the maximum leverage is 25 or more, $5,000,000 from 20x to 25x, $2,000,000 from 10x to 20x and $500,000 below that.
When it is the right tool is a question of what the delay costs. The agent's first step goes at market unless a limit price is named, and that is a measured choice, not a preference: on two years of BTC and ETH, read on 3 October 2026, a resting first step saved the taker's fee and often got a better price but lost 17 to 39% of the attempts, and the net per attempt was best at market everywhere.
Example
An example, in round figures. A market buy of 1 BTC with the mid at $100,000 and a slippage bound of 300 basis points is signed at $103,000: every level up to that price may fill, nothing above it does. If the best ask holds the whole size, the fill is $100,000, the notional $100,000, and the taker's fee at the base rate 0.045% of it, so $45.