The uptick rule was a rule imposed by the Securities and Exchange Commission (SEC) that prevented short sellers to short a stock at a price lower than the last trading price. It was adopted in 1938 and eliminated on July 7, 2007.
The rule stated that every short sale must take place at an "uptick" -- i.e. the price at which the short seller sells the stock must either be above the immediate trading price, or at the last trading price if it were higher than the second last price. This rule made it mechanically impossible for short sellers to drive down stock price by putting in sale orders at progressively lower price.