A securities trade executed on an exchange at the same price as the preceding trade, but at a lower price than the last trade of a different price. For example, if a succession of trades occur in the following order - $10.25, $10.00, and $10.00 - the last trade would be considered a zero minus tick or zero downtick trade.
Until 2007, Securities And Exchange Commission (美国证券交易委员会) regulations prohibited against selling a stock short on a down tick or a zero minus tick. As a result potential short sales would have to pass a tick test to make sure that the stock was trading up or flat before the short sale could go through. The restriction was eliminated after the 美国证券交易委员会 concluded that U.S. markets functioned orderly enough that excessive short sales would not artificially drive down prices. The advent of decimalization also helped the rule get lifted because it reduced the average size of a tick move from fractions to pennies.