A lot is a position-size unit designed to standardise trade volume — in forex, one standard lot is 100,000 units of the base currency, with mini (0.1) and micro (0.01) fractions.

Lot size is the lever that connects a stop-loss distance to a money amount, which makes it the core variable in evaluation risk management: risk per trade equals lot size × pip value × stop distance. Traders who size positions in lots first and check the money risk second routinely violate their own risk plans; the professional order is reversed — define the money risk from your daily limit, then derive the lot size.