A transaction/ trade that takes place at the same price level of a previous transaction/ trade, but in situation where the last recorded move was a minus tick/ downtick. A buy downtick order can be filled and executed only on an downtick or zero minus tick. For such orders, the trade price must always be lower than the last recorded price. This occurs when the execution price is the same as the previous trade, but lower than the most recent trade at a different price. For example, if a series of trades takes place at $25.02, $25.02, and $25.02, the last trade (or tick) is the zero minus tick because it executed at a price equal to that of the previous trade, but lower than the preceding tick of $25.05.

The reverse of a zero minus tick (zero downtick) is a zero plus tick (zero uptick).