Lot
A lot is the unit used to size a position. In forex, 1 standard lot equals 100,000 units of the base currency.
When you place an order, you choose a volume expressed in lots. In forex, 1 standard lot is 100,000 units of the first currency in the pair: buying 1 lot of EUR/USD means buying €100,000 against dollars.
Brokers accept fractions: 0.1 lot (mini), 0.01 lot (micro) and sometimes 0.001 (nano). On index or gold CFDs, lot size varies by broker: 1 lot of gold is often 100 ounces, but not everywhere.
The lot sets the value of each pip, and therefore your real risk. That is why position size is calculated from the stop, never by feel.