A security purchase that occurs at the same time as a previous sale but at a higher price. For example, if a stock sells at $40 and then $40.50 and the next sale is still at $40.50, it is considered to be made at a zero-uptick. Short sales must occur on zero-upticks due to Security and Exchange Commission rules that allow investors to sell short only on a zero-uptick. The purpose of the rule is to prevent a group of linked traders from driving down a stock by heavy short selling and then buying back the shares to reap a large profit. Also called zero-plus tick.