A lot is a unit that measures the standard size of a transaction in forex markets in terms of base currency. Specifically, one lot is 100,000 (hundred thousand) units of the base currency. In the EUR/USD pair a lot is 100,000 euros or its equivalent in dollars.

Before the diffusion of forex trading to private investors/traders the standard transaction used to be one lot but the popularization of forex markets created smaller transaction units such as mini lots (10,000 units of base currency) and micro lots (1,000 units of base currency) or even smaller. This advance makes trading with a small account feasible and less risky.