It's when there's more than you can carry! Just kidding. A "lot" in forex is actually a shorthand way of telling someone how much of a currency you are buying or selling.
One lot is 100,000 units of the base currency. So, if you're trading the USD/JPY currency pair, one lot is $100,000 USD because that is the base currency. If you're trading the EUR/USD pair, one lot is €100,000 EUR.
Remember that because of leverage in Forex, you don not have to have $100,000 in your account to trade one full lot. For more information about this topic, watch the module Lots and Leverage.
For more definitions and trading terms please check out our Glossary