market order

n.
An order to buy or sell stocks or commodities at the prevailing market price.

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 Says:
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, such as Microsoft or Wal-Mart.

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Order to buy or sell a security at the best available price. Most orders executed on the exchanges are market orders.