Stock ahead is a condition that causes an investor’s order not to be executed, even though the stock is trading at a price that would satisfy the customer’s limit order, because other limit orders have been entered prior to the customer’s order.
Applying "Stock Ahead" to Securities Exams:
When customer limit orders are placed for exchange listed securities the orders are executed based on price and time priority. If multiple limit orders to buy or sell at the time price are presented to the market, the order that reached the market first will be executed first. The orders that reached the market first and are being execute prior to the other orders is know as stock ahead.