The uptick rule is a regulation that requires every short sale transaction to be entered on an uptick or zero-plus tick. It aims to prevent short sellers from putting additional downward pressure on a stock that is already experiencing significant losses. Essentially, the rule means that short selling is only allowed on an uptick, i.e., when the price of a stock is higher than the previous trade price. The uptick rule was implemented by the SEC in 1938 but was repealed in 2007.