What is Stock Ahead?

When two or more orders for a stock at a certain price arrive about the same time, and the exchange's priority rules take effect. NYSE rules stipulate that the bid made first should be executed first, or, if two bids come in at once, the bid for the larger number of shares receives priority. The bid that is not executed is then turned to the broker, who informs the customer that the trade was not completed because there was "stock ahead.".