Also a market order, but the investor is allowing the floor broker who is executing the order to use his own discretion as to the exact timing of the execution. If the floor broker expects a decline in price and he is holding a "market not held buy order", he (she) may wait to buy, figuring that a better price will soon be available. There is no guarantee that a "market not held order" will be filled.
See Also:
Market: A specific asset, security or commodity that is traded at an exchange.
Order: A ticket or voucher representing long or short securities and options.