Soemtimes called instant orders, an order that lets a trader buy or sell a cryptocurrency immediately. When a market order is placed, a trader doesn't have to wait for the asset he's buying or selling to reach a certain price. Market orders don't guarantee the exact price that you see on the screen, but market orders are the fastest way to buy or sell
Example
“A trader placed a market order to buy 1 BTC at the current price of $30,000 and was filled instantly at $30,012 due to slight slippage during high volatility.”