FINRA rules define a "pattern day trader" as any customer who executes 4 or more "day trades" within 5 business days, provided that the number of day trades represents more than 6% of the customer’s total trades in the margin account for that same 5 business day period.  This rule represents a minimum requirement, and some broker-dealers use a slightly broader definition in determining whether a customer qualifies as a "pattern day trader." 

e.g. - a B/D also may designate a customer as a "pattern day trader" if it "knows or has a reasonable basis to believe" that a customer will engage in pattern day trading.  A customer who receives day trading training from the B/D, before opening the account, could be designated as a "pattern day trader." 

FINRA rules define a "day trade" as the purchasing and selling or the selling and purchasing of the same security on the same day in a margin account.  This definition encompasses any security, including options.  Selling short and purchasing to cover a position in the same security on the same day is also considered a day trade.  Exceptions to this definition include:

• a long security position held overnight and sold the next day prior to any new purchase of the same security; or
• a short security position held overnight and purchased the next day prior to any new sale of the same security.

    SEC Advice and Resources.   The SEC advises customers to check with their brokerage firms to learn whether their trading activities will cause them to be designated as pattern day traders.  Under FINRA rules, customers who are deemed "pattern day traders" must have at least $25,000 in their accounts and can only trade in margin accounts.

For further details, refer to FINRA rules, go to these SEC resources:  [SEC: Pattern Day Trader, SEC: Day Trade, SEC: Margin Rules for Day Trading, updated 2/10]