Define 'market orders' in trading.

Market orders are defined as orders to buy or sell a security immediately at the best available price. This type of order is executed as quickly as possible at the current market price, making it a straightforward way for traders to enter or exit positions without delay.

Traders commonly use market orders when they want to ensure that their order is filled quickly, particularly in liquid markets where prices can change rapidly. The main characteristic of a market order is that it prioritizes execution over price, meaning that the trader accepts the price available at the moment the order is placed. This immediacy often leads to a prompt transaction, though the exact price received may differ from what the trader anticipated, especially in volatile conditions.

The other options do not capture the essential nature of market orders. For instance, the option related to holding a security until the price changes refers more closely to limit orders or conditional orders rather than market orders, while waiting for a predetermined time period also does not relate to the immediate execution purpose of market orders. Additionally, the requirement for approval from a trading supervisor indicates an administrative process not inherent to how market orders function.