Market Order
An order that executes immediately at the best available price — you pay for speed with the spread and slippage.
A market order buys or sells instantly against the best resting offers in the book. Certainty of execution is its whole product; price is the payment: you cross the spread, and on large orders or thin books each successive fill is worse — slippage.
On deep pairs like BTC/USDT the cost is trivial for small orders; on small-cap tokens a market order can move price several percent against you before completing. Rule of thumb: market orders when time matters more than a few basis points, limit orders otherwise — and never a market order into visibly thin liquidity.