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 units (mini lots), 1,000 unit (micro lots), or 100 (nano lots) meaning buying / selling 100,000 of the base currency while selling/buying the equivalent number of units of the counter currency.

For example, if you open a long position of one lot for EUR/USD for the ask price of 1.4000, you are purchasing 100,000 Euro while, selling 140,000 USD.

A standard contract (one Lot) in which the USD is the counter currency one pip will equal 10 USD (1 USD for a mini lot). For all other pairs, exact pip values are slightly different and range from 8 USD to 10 USD.