Lot

Lot is a standard trading term referring to an order of 100,000 units. There are also mini, micro, and nano lot sizes that are 10,000, 1,000, and 100 units, respectively.

Currency pairs are usually traded in units of 100,000 (standard lots), 10,000 (mini lots), 1,000 (micro lots), or 100 (nano lots), meaning buying or selling 100,000 of the base currency while selling or buying the equivalent number of units of the counter currency.

For example, if you open a long position of one lot for EUR/USD at the ask price of 1.4000, you are purchasing 100,000 euros while selling 140,000 USD.
In a standard contract (one lot), where the USD is the counter currency, one pip will equal 10 USD (or 1 USD for a mini lot). For all other pairs, the exact pip values are slightly different and range from 8 USD to 10 USD.