Lot
A standardised position size. 1 standard lot = 100,000 units of the base currency.
Three sizes: standard (1.00 = 100,000 units), mini (0.10 = 10,000), micro (0.01 = 1,000). On EURUSD, 1 standard lot moves roughly $10 per pip. Never pick lot size first - work backwards from your % risk and stop distance.
Related
Learn to actually use Lot.
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.