A Securities and Exchange Commission trading regulation that restricted short sales of stock from being placed on a downtick in the market price of the shares. Short sales could only be permitted on upticks or zero-plus ticks . The regulation was passed in 1938 to prevent selling shares short into a declining market; at the time market mechanisms and liquidity couldn't be guaranteed to prevent panic share declines or outright manipulation.
This regulation was rescinded in July 2007 by decree of the SEC; as a result short sales can occur on any price tick in the market, whether up or down.
The short sale rule was also known as the "plus-tick rule", "tick-test rule", or "uptick rule".
Taobiz explains Short-Sale Rule
The SEC began examining the possibility of eliminating this short-sale rule following the decimalization of the major stock exchanges in the early 2000s. Because tick changes were shrinking in magnitude following the change away from fractions, and U.S. stock markets had become more stable, it was felt that the restriction was no longer necessary.
The SEC ran a test program of stocks in 2003 to see if removing the short-sale rule would have any negative effects. After reviewing the results it was decided that the rule no longer needed to exist. However, naked shorting - selling shares short that don't exist or can't be verified - is still illegal.