A simple purchase or sale on an exchange at the current price. Market buys purchase the cheapest asset available on the order book, and the market sells fill the most expensive buy order on the books.
A market order is the most simple order type and is sometimes preferred because it guarantees it will be executed. If an investor isn’t concerned with a small movement in price from the quote to the execution a market order makes perfect sense.
On the other hand, if you’re buying an asset with a small trading volume it might be best to avoid a market order because the lack of liquidity often means a huge spread between the quoted price and the actual fill. Spreads can also increase on market orders during times of high volatility in the market.