What defines a stop order?

A stop order, also known as a stop-loss order, is defined as an order that is not executed until a predetermined price is reached. Once the security's price hits this specified level, the stop order becomes a market order, which means it is executed at the next available market price. This mechanism is particularly useful for traders aiming to limit potential losses or lock in profits without having to constantly monitor the market.

The other options address different types of orders or actions in trading. For instance, an order to sell at any price does not align with the concept of a stop order since a stop order specifically involves a trigger price. Similarly, an order executed at the market price indicates an immediate order type rather than a condition-based one like a stop order. Lastly, halting all trading of a specific security refers to a regulatory action rather than an instruction given by a trader concerning their securities. Thus, the chosen definition accurately captures the essence and functionality of a stop order in trading.