Market Order

What Does Market Order Mean?
An order to buy or sell a stock immediately at the best available current price.

A market order is 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.