Definition of 'Market Order'
An order that an investor makes through a broker or brokerage service to buy or sell an investment immediately at the best available current price. A market order is the default option and is likely to be executed because it does not contain restrictions on the buy/sell price or the timeframe in which the order can be executed.
A market order is also sometimes referred to as an "unrestricted order."
Investopedia explains 'Market Order'
A market order guarantees execution, and it often has low commissions due to the minimal work brokers need to do. Be wary of using market orders on stocks with a low average daily volume: in such market conditions the ask price can be a lot higher than the current market price (resulting in a large spread). In other words, you may end up paying a whole lot more than you originally anticipated! It is much safer to use a market order on high-volume stocks.