Position size
The lot size of a trade, calculated from % risk and stop distance - NOT picked by vibes.
Formula: (account x risk%) / (stop pips x pip value). $10,000 x 1% / ($10 x 30 pips) = 0.33 lots. Ignoring this formula is the single most expensive retail mistake. Use a position size calculator, always.
Learn to actually use Position size.
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.