A lot is a standard size of a transaction.
1 lot or a Standard lot = 100 000 units of the base currency in a currency pair.
If we take EUR/USD for example, 1 lot will be 100 000 units of EUR.
Other volume related terms:
* 0.1 lots or a Mini lot = 10 000 units;
* 0.01 lots or a Micro lot = 1 000 units.