What Is Market order?
A market order is a trade where a trader order is instantly fulfilled whatever is the availability liquidity and price at the markets.
Specifically
Market orders remove liquidity (also known as a market taker)
Market buys move price up, market sells move price ddown
The opposite of a market order is often a limit order. On AMMs the opposite is liquidity provider position.
On AMM decentralised exchanges every swap is a similar to a market order.
Market orders, as a market taker, typically have higher fees than market makers (provides liquidity), as liquidity is the key selling point for any exchange.
See also