A situation in which an order is placed, but not executed, because of a previously sent order involving the same price. Depending on the exchange's priority rules, this can also happen when two bids are made at the same time with identical prices; only the larger order will be executed.
Taobiz explains Stock Ahead
For example, if two orders - one for 1,000 shares, the other for 500 shares - are placed for XYZ stock at the same price, the order for 1,000 shares would be executed and the order for 500 shares would not be made because there was a "stock ahead".
