Uptick Rule. What Does It Mean?

Uptick Rule
What Does It Mean? (Source Investopedia)
What Does Uptick Rule Mean?
A former rule established by the SEC that requires that every short sale transaction be entered at a price that is higher than the price of the previous trade. This rule was introduced in the Securities Exchange Act of 1934 as Rule 10a-1. The uptick rule prevents short sellers from adding to the downward momentum when the price of an asset is already experiencing sharp declines. The SEC eliminated the rule on July 6, 2007.

The uptick rule was also be known as the "plus tick rule".
Investopedia Says
Investopedia explains Uptick Rule
By entering a short sale order with a price above the current bid, a short seller ensures that his or her order is filled on an uptick. The uptick rule is disregarded when trading some types of financial instruments such as futures, single stock futures, currencies or market ETFs such as the QQQQ or SPDRs. These instruments can be shorted on a downtick because they are highly liquid and have enough buyers willing to enter into a long position, ensuring that the price will rarely be driven to unjustifiably low levels.

S&P jumps 5 percent after uptick rule talk

NEW YORK (Reuters) - Stocks jumped on Tuesday, extending strong gains after Rep. Barney Frank said he expects the SEC's uptick rule to be restored in about a month.The uptick rule, which only allowed a stock to be sold short when the last sale price was higher than the previous price, was repealed by the SEC in 2007 because the agency found that changes in trading strategies made it ineffective.
Frank added that mark-to-market accounting rule must be improved and made more flexible. A congressional panel is set to conduct a hearing on Thursday.
The Dow Jones industrial average (DJI:^DJI - News) jumped 326.72 points, or 4.99 percent, to 6,873.77. The Standard & Poor's 500 Index (^SPX - News) rallied 38.42 points, or 5.68 percent, to 714.95. The Nasdaq Composite Index (Nasdaq:^IXIC - News) shot up 77.19 points, or 6.08 percent, to 1,345.83.