PDT (Pattern Day Trader Rule)
US SEC rule: if you have under $25,000 in a margin account, you're limited to 3 day trades per rolling 5 business days.
Applies to US stocks + options, not forex. Hit 4 day trades in 5 days with <$25K and your account is flagged - restricted to cash-only for 90 days. Forex has no PDT rule - one reason beginners gravitate to forex when undercapitalised for stocks.
Related
Learn to actually use PDT.
Definitions are the easy part. The free first five modules put this on a real chart and make you do the work. No card required.