On February 27th, 2001, the Securities and Exchange Commission (SEC) approved amendments to NASD Rule 2520. These amendments modify margin requirements for individuals who actively daytrade stocks and become effective on September 28th, 2001.
Your broker and/or the NASD will consider you a "pattern daytrader" if you buy and sell (or short and cover) any security on the same day in a margin account AND you execute four (4) or more such trades during any rolling five business day period (Saturday, Sunday and market holidays do not impact the five day rolling period).
If, at any time, you execute four (4) or more such trades in a rolling five day period, you'll then be required to maintain a minimum equity of $25,000 in your margin account prior to any further day-trading activities.